rtp_engine: Remove the double assigned RTP payload ID of H.263+.

Mantis-3709 (Commit 68ff3c35, Asterisk 1.2) added support for the video format
H.263+. For this, the RTP payload ID 103 got assigned statically. Commit f1aadc81
assigned another payload ID 98 for this format in Asterisk 1.6.

Change-Id: I90e35b158487f8f1f8187da6241b54cd3b74e667