Use SILK/CELT everywhere describing modes.

There was some attempt to use LP/MDCT instead, to avoid confusion
of the Opus modes with the earlier codecs of the same name, but
Jean-Marc says they gave up on doing that in the Opus RFC, and in
particular the tables a reader would need to reference from RFC 6716
Section 3.2 mentions SILK and CELT, so I think it's important to use
the same terms here.
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