clients: drop simple-dmabuf-drm

clients: drop simple-dmabuf-drm

This client contains driver-specific code to allocate buffers. However clients
shouldn't contain driver-specific code and should rely on e.g. mesa to allocate
buffers via standard interfaces.

Additionally, because the build system always tries to enable all features, some
experimental drivers and drivers that aren't included in amd64 distribution
packages were required. Users would need to manually disable some drivers.
Releasers would need to install libdrm from source (because the release process
forbids adding custom build flags). Dropping simple-dmabuf-drm simplifies both
building and releasing.

The functionality previously tested via simple-dmabuf-drm can now be tested with
simple-dmabuf-egl.

Signed-off-by: Simon Ser <contact@emersion.fr>