Delete unused SdpSemantics attribute from RTCConfiguration.

Plan B has been unavailable on most platforms since M102, on CrOS since
M104 and Fuchsia since M105.

Even though SdpSemantics was a NO-OP, it was still listed in the
RTCConfiguration in order to expose the value "unified-plan" and to
avoid TypeError.

In this CL we remove it from WebIDL. Setting SdpSemantics will still not
give a TypeError since unexisting attributes are ignored by WebIDL, so
the only difference is that one can no longer do
"pc.getConfiguration().sdpSemantics" and read out "unified-plan". This
is compatible with other Unified Plan-only browsers (Firefox/Safari)
where sdpSemantics never existed so we do not expect any compatibility
issues with removing this non-standard attribute.

Bug: 1354456
Change-Id: Ifab4006b2e8cf3dac8f39490ed484a15a52dce66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3890944
Commit-Queue: Henrik Boström <hbos@chromium.org>
Auto-Submit: Henrik Boström <hbos@chromium.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/main@{#1047482}