Drop support for zstandard

[crusaderky]

Following A/B tests con Coiled, zstandard seems to be, consistently, a poorer choice than lz4.
I do not see any reason why one would prefer it to lz4 andI think we should drop it in order to reduce our support surface.

A/B tests of zstandard vs. lz4, used for both network comms and spilling, exhibit a 15~80% slowdown in end-to-end runtime across the board:

XREF #2970
XREF #7403

[fjetter]

Is zstd actually causing problems? I don't see the benefits of removing it.

[crusaderky]

Is zstd actually causing problems? I don't see the benefits of removing it.

It isn't; it's just useless. It would be one less moving part to maintain.