drm/doc: internals: remove section on PCI legacy support

The functions that were described in this section of
drm-internals.rst were removed in NOV-2023, along with all of the
kernel-doc comments in the source file. This now causes a
docs build warning, so remove that section of the documentation also.

drivers/gpu/drm/drm_pci.c:1: warning: no structured comments found

Fixes: 2504c7ec

 ("drm: Remove source code for non-KMS drivers")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240112051731.15722-1-rdunlap@infradead.org