remove buck2 from runtime overview (#4219) (#4251)

Summary:
Pull Request resolved: #4219

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 from runtime overview.

Reviewed By: kirklandsign

Differential Revision: D59646100

fbshipit-source-id: c7ebe6f4cd2615036dd0d9a6dff30cae6684986d
(cherry picked from commit 1c42a7f)

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

Author: pytorchbot

docs/source/runtime-overview.md

@@ -101,8 +101,7 @@ can build it for a wide variety of target systems.
   to them.
 * The code is compatible with GCC and Clang, and has also been built with
   several proprietary embedded toolchains.
-* The repo provides both CMake and buck2 build systems to make integration
-  easier.
+* The repo provides CMake build system to make integration easier.
 
 #### Operating System Considerations