remove buck2 from cmake installation tutorial (#4214) (#4253)

Summary:
Pull Request resolved: #4214

For alpha+, we need to remove all buck2 commands and buck2 dependencies from static doc and github readmes.

 This diff gets rid of the buck2 related commands and installations from cmake set up page

Reviewed By: kirklandsign

Differential Revision: D59643087

fbshipit-source-id: 39764a4a46f729d4414b86213b4d73242abbbd45
(cherry picked from commit d171297)

Co-authored-by: Songhao Jia <[email protected]>

Author: pytorchbot

docs/source/runtime-build-and-cross-compilation.md

@@ -1,16 +1,13 @@
 # Building with CMake
 
-Although buck2 is the main build system for the ExecuTorch project, it's also
-possible to build core pieces of the runtime using [CMake](https://cmake.org/)
-for easier integration with other build systems. Even if you don't use CMake
-directly, CMake can emit scripts for other format like Make, Ninja or Xcode.
-For information, see [cmake-generators(7)](https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html).
+ExecuTorch uses [CMake](https://cmake.org/)  as its primary build system.
+Even if you don't use CMake directly, CMake can emit scripts for other format
+like Make, Ninja or Xcode. For information, see [cmake-generators(7)](https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html).
 
 ## Targets Built by the CMake Build System
 
-ExecuTorch's CMake build system doesn't cover everything that the buck2 build
-system covers. It can only build pieces of the runtime that are likely to be
-useful to embedded systems users.
+ExecuTorch's CMake build system covers the pieces of the runtime that are
+likely to be useful to embedded systems users.
 
 - `libexecutorch.a`: The core of the ExecuTorch runtime. Does not contain any
   operator/kernel definitions or backend definitions.
@@ -31,17 +28,8 @@ useful to embedded systems users.
 
 Follow the steps below to have the tools ready before using CMake to build on your machine.
 
-1. Clone the repo and install buck2 as described in the "Building a Runtime" section
-   of [Setting Up ExecuTorch](getting-started-setup.md#building-a-runtime)
-   - `buck2` is necessary because the CMake build system runs `buck2` commands
-     to extract source lists from the primary build system. It will be possible
-     to configure the CMake system to avoid calling `buck2`, though.
-2. If your system's version of python3 is older than 3.11:
+1. If your system's version of python3 is older than 3.11:
    - Run `pip install tomli`
-   - This provides an import required by a script that the CMake build system
-     calls to extract source lists from `buck2`. Consider doing this `pip
-     install` inside your Python or Conda virtual environment if you created
-     one already by following [Setting up ExecuTorch](getting-started-setup.md#setting-up-executorch).
 3. Install CMake version 3.19 or later:
    - Run `conda install cmake` or `pip install cmake`.
 
@@ -57,9 +45,6 @@ cd executorch
 
 # Clean and configure the CMake build system. It's good practice to do this
 # whenever cloning or pulling the upstream repo.
-#
-# NOTE: If your `buck2` binary is not on the PATH, you can change this line to
-# say something like `-DBUCK2=/tmp/buck2` to point directly to the tool.
 (rm -rf cmake-out && mkdir cmake-out && cd cmake-out && cmake ..)
 ```
 
@@ -140,7 +125,6 @@ Assuming Android NDK is available, run:
 rm -rf cmake-android-out && mkdir cmake-android-out && cd cmake-android-out
 
 # point -DCMAKE_TOOLCHAIN_FILE to the location where ndk is installed
-# Run `which buck2`, if it returns empty (meaning the system doesn't know where buck2 is installed), pass in this flag `-DBUCK2=/path/to/buck2` pointing to buck2
 cmake -DCMAKE_TOOLCHAIN_FILE=/Users/{user_name}/Library/Android/sdk/ndk/25.2.9519653/build/cmake/android.toolchain.cmake  -DANDROID_ABI=arm64-v8a ..
 
 cd  ..