Documentation/i915: remove sphinx conversion artefact

Remove old warning about docproc directive that's not supported in the
Sphinx toolchain.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7fc8a110b78a9dc9a585dce643b68b4200b7e793.1502368010.git.jani.nikula@intel.com