Docs: do not mention python.system_packages

This option doesn't work on builds using `build.os` because the Python binary
used it's not the one from the system, but another one compiled via `asdf`.
This means that creating a virtualenv with access to the "system packages" has
no effect at all.

As a first step, I'm removing this option from the documentation to avoid
confusions. Next, we should probably our deprecation policy to contact users
ands  remove it from the code as well.

Closes #10500

Author: humitos

docs/user/config-file/v2.rst

@@ -130,7 +130,6 @@ Configuration of the Python environment to be used.
            - docs
        - method: pip
          path: another/package
-     system_packages: true
 
 python.version
 ``````````````
@@ -228,20 +227,6 @@ With the previous settings, Read the Docs will execute the next commands:
 
    pip install .[docs]
 
-python.system_packages
-``````````````````````
-
-Give the virtual environment access to the global site-packages directory.
-
-:Type: ``bool``
-:Default: ``false``
-
-.. warning::
-
-  If you are using a :ref:`Conda <config-file/v2:conda>` environment
-  to manage the build, this setting will not have any effect, since
-  the virtual environment creation is managed by Conda.
-
 conda
 ~~~~~
 
@@ -868,8 +853,6 @@ Changes
 - The settings ``python.setup_py_install`` and ``python.pip_install`` were replaced by ``python.install``.
   And now it accepts a path to the package.
   See :ref:`config-file/v2:Packages`.
-- The setting ``python.use_system_site_packages`` was renamed to ``python.system_packages``.
-  See :ref:`config-file/v2:python.system_packages`.
 - The build will fail if there are invalid keys (strict mode).
 
 .. warning::