drm/doc/rfc: Removing missing reference to xe.rst

The file has already been deleted as the tasks were completed.
However the index reference was missed behind.

fixes a doc build warning:

Documentation/gpu/rfc/index.rst:35: WARNING: toctree contains
reference to nonexisting document 'gpu/rfc/xe'

Fixes: d11dc7aa ("drm/doc/rfc: Remove Xe's pre-merge plan")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240116220331.145607-1-rodrigo.vivi@intel.com