drm/nouveau: Remove support for legacy contexts/buffers

Remove nouveau's support for legacy contexts and buffers. It was
required by libdrm earlier than 2.4.33, released in March 2012. A
previous attempt in 2013 to remove the functionality [1] had to be
reverted [2] as there were still users left. Libdrm 2.4.33 is now
almost 11 years old and it is time for userspace to move on.

With the nouveau code gone, we can also remove the driver-feature
bit DRIVER_KMS_LEGACY_CONTEXT.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c510133d93dd6f15ca040733ba7b2891ed61fd1 # 1
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c21eb21cb50d58e7cbdcb8b9e7ff68b85cfa5095 # 2
Link: https://patchwork.freedesktop.org/patch/msgid/20230112133858.17087-1-tzimmermann@suse.de