Remove original Biquad type from synthio

[jepler]

In #9804 we deprecated the original synthio biquads in favor of the new BlockBiquads. For version 10, make the incompatible change of actually removing the old biquads.

[dhalbert]

@relic-se I started to work on this, but I'm unfamiliar with the code. Re the struct biquad_filter_state and the associated routines in synthio/Biquad.h: should these be moved to BlockBiquad, or are they completely unnecessary? The are used in audiofilters/Filter.c, synthio/Note.c etc.

Also, is the AnyBiquad type now unneeded? Are they going to be a BlockBiquad? Should we consider renaming BlockBiquad to just Biquad, deprecating BlockBiquad, and removing the old name in CPy 11?

If you have a clear idea of the changes for this issue, and could do a PR, that would be great. Thanks.

[relic-se]

I'll need to do some more digging, but I am definitely in favor of renaming BlockBiquad to Biquad. And you are correct on AnyBiquad which is just an amalgamation of the old and new classes.

I'd be happy to work on the PR here! I'll see if I can get something drafted this weekend.