Changes

trau cosmetic: update osmo_trau2rtp API comments

Since the writing of Doxygen API comments for osmo_trau2rtp() and
osmo_trau2rtp_ufe(), these APIs gained the ability to convert CSD
TRAU frames to TW-TS-007 in addition to CLEARMODE - document this
addition.  Also update comments regarding AMR, which is now supported
via AMR TRAU frame interworking facility with its own API.

Change-Id: Id287474c5824ab334da244701700051f9f014e66

trau2rtp: rm defunct AMR WIP code

When the present TRAU<->RTP conversion module was initially introduced
in 2020, there was a WIP plan to eventually extend it to support AMR
in addition to FR/HR/EFR.  Defunct code for a beginning of trau2rtp
implementation for AMR has sat in trau_rtp_conv.c under '#if 0' all
these years - but it never became anything beyond dead code.  When
the time came to implement AMR TRAU support for real, the existing
TRAU<->RTP API from 2020 design turned out to be insufficient; the
AMR support that was merged just before 2025-12 major release takes
the form of an entirely new AMR TRAU frame interworking facility
with its own API.  As a result of these developments, the original
defunct AMR code from 2020 now only adds confusion - remove it.

Change-Id: Ieeee8eb6dbf9c753458a91e13f2bf22311936259

trau_rtp cosmetic: rm dead const lines

The two const data items removed in this patch were already commented
out, and it is not clear where or how they were ever intended to be
used.  Remove them to reduce clutter.

Change-Id: Ic1b091ec96eafc8759d90d14201c8698ef251761