[net processing] Remove fSupportsDesiredCmpctVersion and simplify sendcmpct processing

Summary:
fSupportsDesiredCmpctVersion was never useful for Bitcoin ABC, as we support only a single version for compact blocks.

Backport the relevant part of core#20799 to make it go away and simplify the code.
https://github.com/bitcoin/bitcoin/pull/20799/commits/16730b64bb4a11995d792f626c8a63318e5eaeeb
https://github.com/bitcoin/bitcoin/pull/20799/commits/42882fc8fc2ef5c58eb963f7f1e852dd43de6c65
https://github.com/bitcoin/bitcoin/pull/20799/commits/25edb2b7bd4c41156fba09d0033a978e362435af
https://github.com/bitcoin/bitcoin/pull/20799/commits/a45d53cab556505048c387429fd07188e4c40c3d
https://github.com/bitcoin/bitcoin/pull/20799/commits/30c3a01874cf51d987a0ae2bb699bf50d82768ff
https://github.com/bitcoin/bitcoin/pull/20799/commits/d0e97741748aaaad2a89ca42e4898e7f01308b35

Depends on D14872

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14874