Buffers containing frame data received by mutter always contain buffers
of type MemFd or DmaBuf.
So, remove the path that ignores the buffer data, as it is supposed to
be never hit.

Buffers containing frame data received by mutter always contain buffers
of type MemFd or DmaBuf.
So, remove the path that ignores the buffer data, as it is supposed to
be never hit.

This only leads to an assertion hit. If it would not hit the assertion
in on_frame_ready, it would leak the frame.

Fixes: https://errors.ubuntu.com/problem/6f9ac35833672d86f0f7a13a9a68551f186a55f2