remove check hooks for GUCs that contribute to MaxBackends

While working on an idea from another thread [0], I noticed that each of
max_connections, max_worker_process, max_autovacuum_workers, and
max_wal_senders have a check hook that verifies the sum of those GUCs does
not exceed a certain value.  Then, in InitializeMaxBackends(), we do the
same check once more.  Not only do the check hooks seem redundant, but I
think they might sometimes be inaccurate since some values might not yet be
initialized.  Furthermore, the error message is not exactly the most
descriptive:

    $ pg_ctl -D . start -o "-c max_connections=262100 -c max_wal_senders=10000"

    FATAL:  invalid value for parameter "max_wal_senders": 10000

The attached patch removes these hooks and enhances the error message to
look like this:

    FATAL:  too many backends configured
    DETAIL:  "max_connections" (262100) plus "autovacuum_max_workers" (3) plus "max_worker_processes" (8) plus
"max_wal_senders"(10000) must be less than 262142.
 

The downside of this change is that server startup progresses a little
further before it fails, but that might not be too concerning given this
_should_ be a relatively rare occurrence.

Thoughts?

[0] https://postgr.es/m/20240618213331.ef2spg3nasksisbi%40awork3.anarazel.de

-- 
nathan