Remove mention of manyFiles git option (spack#50874)

Since we only have ~1700 files, which isn't that many.

Author: haampie

README.md

@@ -53,19 +53,9 @@ To install spack, first make sure you have Python & Git.
 Then:
 
 ```bash
-git clone -c feature.manyFiles=true --depth=2 https://github.com/spack/spack.git
+git clone --depth=2 https://github.com/spack/spack.git
 ```
 
-<details>
-<summary>What are <code>manyFiles=true</code> and <code>--depth=2</code>?</summary>
-<br>
-
-> `-c feature.manyFiles=true` improves git's performance on repositories with 1,000+ files.
->
-> `--depth=2` prunes the git history to reduce the size of the Spack installation.
-
-</details>
-
 ```bash
 # For bash/zsh/sh
 . spack/share/spack/setup-env.sh

lib/spack/docs/getting_started.rst

@@ -13,7 +13,7 @@ Getting Spack is easy.  You can clone it from the `GitHub repository
 
 .. code-block:: console
 
-   $ git clone -c feature.manyFiles=true --depth=2 https://github.com/spack/spack.git
+   $ git clone --depth=2 https://github.com/spack/spack.git
 
 This will create a directory called ``spack``. Once you have cloned Spack, we recommend sourcing the appropriate script for your shell:
 
@@ -40,8 +40,6 @@ This will create a directory called ``spack``. Once you have cloned Spack, we re
 That's it! You're ready to use Spack.
 
 .. note::
-   ``-c feature.manyFiles=true`` improves Git's performance on repositories with 1,000+ files.
-
    ``--depth=2`` prunes the git history to reduce the size of the Spack installation.
 
 -------------

lib/spack/docs/index.rst

@@ -34,13 +34,11 @@ package:
 
 .. code-block:: console
 
-   $ git clone -c feature.manyFiles=true --depth=2 https://github.com/spack/spack.git
+   $ git clone --depth=2 https://github.com/spack/spack.git
    $ cd spack/bin
    $ ./spack install libelf
 
 .. note::
-   ``-c feature.manyFiles=true`` improves Git's performance on repositories with 1,000+ files.
-
    ``--depth=2`` prunes the git history to reduce the size of the Spack installation.
 
 If you're new to Spack and want to start using it, see :doc:`getting_started`,