Removed a useless called to abs.

Clang 3.5 reported the following error: error: taking the absolute
value of unsigned type 'unsigned int' has no effect
[-Werror,-Wabsolute-value]

Subtracting unsigned variables will never be negative and will either
be the small expected value or will wrap to a very big value. This
code is trying to determine if the difference between these timestamps
is greater than 16000.

The variables last_write_ts and this_ts deal with timestamps. In the
normal case this_ts will be a larger timestamp than
last_write_ts. This change will maintain the intended behavior of
reseting the video if the difference is larger than
16000 and in the abnormal case this value would wrap and still exceed
the 16000.

src/switch_rtp.c

@@ -6341,7 +6341,7 @@ static int rtp_common_write(switch_rtp_t *rtp_session,
 	if (!switch_rtp_test_flag(rtp_session, SWITCH_RTP_FLAG_VIDEO)) {
 		this_ts = ntohl(send_msg->header.ts);
 
-		if (abs(rtp_session->last_write_ts - this_ts) > 16000) {
+		if ((this_ts - rtp_session->last_write_ts) > 16000) {
 			rtp_session->flags[SWITCH_RTP_FLAG_RESET] = 1;
 		}