[rbp/omxplayer] Remove unused state from OMXClock

m_has_audio/m_has_video are always set before OMXReset is called, so just make them local parameters.
m_video_start and m_audio_start are unused, so remove them.

This commit should have no functional change
Merge pull request #2949 from popcornmix/remove_has_video

[rbp/omxplayer] Remove unused state from OMXClock