flvmux: Don't set DELTA_UNIT flag on audio buffer for non-audio only case

Submitted by Seungha Yang

Link to original bug (#796969)

Description

Unset DELTA_UNIT on audio buffer could make sense, however,
if downstream is waiting keyframe, the downstream could be confused,
That is, downstream cannot know which buffer is a video keyframe,
because flvmux unset DELTA_UNIT flag on both keyframe and audio buffer.

Since the original intention of the code was
"mark the buffer if it's an audio buffer and there's also
video being muxed or it's a video interframe", rollback to previous
behavior before the commit 8b814f63.