Changes

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