[codec] Remove OMX-related stuff

This removes the prototype/experimental OMX-based AAC decoder and OMX
runner isolate.

This does not remove the libFDK build target as we'll likely still use
that directly.

We don't need these things now that we have ffmpeg-based SW video decode
and will soon enough have libFDK audio decode.

Tested: CQ
Change-Id: I5333dfadf165a2e5e4b6f5a8c4682aa522259733