VCDIFF

The patch subset method currently specifies two options for compression of
the patch: VCDIFF and shared brotli. Shared brotli produces smaller patches
than VCDIFF. Originally VCDIFF was included as an option since shared
brotli was in it's early days so there wasn't a public implementation or
finished specification. The idea being that if shared brotli wasn't ready
yet implementations could be built with VCDIFF instead.

However, that's since changed and we have a open source implementation of
shared brotli now, and we have a workaround for the specification being a
draft (see: https://github.com/w3c/IFT/issues/101#issuecomment-1251591398).
Also, we've demonstrated that shared brotli is sufficiently performant for
use (see:
https://docs.google.com/document/d/1MdtB_WPC2grAx3vFgLHA1-CqRzQtWj_cJYx40W2VQgI/edit#heading=h.dy3wd9u6su7
).

Given all that I wonder if there's still any value in keeping VCDIFF as an
option in the specification? The advantage of removing it would be reduced
complexity of implementing the specification (both client and server) since
currently VCDIFF support is mandatory. Let's discuss this during the next
working group call.