rtsp: remove CHECK to ignore unsupported tracks

RFC 3551 specifies a number of static A/V profiles for media
payload. These don't use the 'rtpmap' or 'fmtp' properties in the
session description, and thus need to be handled differently.
For now, we just ignore these types as they are very rarely used.

The previous paragraph was taken from
https://android-review.googlesource.com/#/c/240630/,
author David Williams <david.williams@sonymobile.com>.

Bug: 29493841
Change-Id: I12581447f18344ac503fe427e3c940a85d44644e