ext/x264/gstx264enc.*: Do not deal with duplicated input (timestamps).  If needed, a generic element can do so.

Original commit message from CVS:
* ext/x264/gstx264enc.c: (gst_x264_enc_reset),
(gst_x264_enc_chain), (gst_x264_enc_encode_frame):
* ext/x264/gstx264enc.h:
Do not deal with duplicated input (timestamps).  If needed,
a generic element can do so.
Do not manipulate input timestamps on the way out,
since that shifts the timeline and A/V sync.