v4l2_codec2: Remove video_encode/decode_accelerator files.

This CL cleans up the video_encode_accelerator.h/cc and
video_decode_accelerator.h/cc files. These files were copied from
Chrome to help with the development of the V4L2 encoder and decoder,
but most off their functionality has already been removed in a previous
CL. The remaining functionality is integrated into the V4L2Device so
these files can be removed now.

Bug: 155138142
Test: arc.VideoEncodeAccel.h264_192p_i420_vm
Change-Id: I4591e3d03f5ebc6d568cc718ccde5df05af37b36