Changes

rtp2trau HR: remove broken TRAU-16k-UL support

GSM 08.61 defines two TRAU frame formats for HRv1 codec, using
either 16 kbit/s or 8 kbit/s submultiplexing.  The 16k format may be
used only on Abis, not in TFO.  When we interface to an E1 BTS, we
need to receive TRAU-UL frames and generate TRAU-DL frames - not
any other combination.  Untested support for emitting both
TRAU-16k-DL and TRAU-16k-UL frames for HR codec was added in the
very initial version of trau_rtp_conv.c module in 2020 by HW - but
while TRAU-16k-DL output support is potentially useful (if there
exist any historical BTS models that use this format), there is no
code anywhere in Osmocom that uses osmo_rtp2trau() to generate
TRAU-16k-UL frames for HR.

The legacy code that is removed in this patch should be considered
broken.  Proper generation of TRAU-16k-UL frames (e.g., in a BTS
emulator) would require ability to generate BFI, UFI, invalid SID,
TAF and other possible happenings in the UL of a call leg, but no
such capabilities exist in the legacy code being removed.  We do
support all of these capabilities for TRAU-8k-UL output, using
TW-TS-002 as the matching RTP format, but TW-TS-002 was designed
to feature-match TRAU-8k-UL, not TRAU-16k-UL.

Change-Id: If50036c4de9a11db524abffcd87d053878104982