DRM: delete DRM IRQ legacy midlayer docs

DRM: delete DRM IRQ legacy midlayer docs

Remove documentation associated with the removal of the DRM IRQ legacy
midlayer.

Eliminates these documentation warnings:

../drivers/gpu/drm/drm_irq.c:1: warning: 'irq helpers' not found
../drivers/gpu/drm/drm_irq.c:1: warning: no structured comments found

Fixes: c1736b9008cb ("drm: IRQ midlayer is now legacy")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: dri-devel@lists.freedesktop.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005025312.20913-1-rdunlap@infradead.org
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>