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