Fix initdb's handling of min_wal_size and max_wal_size.

Fix initdb's handling of min_wal_size and max_wal_size.

In commit 3e51b278d, I misinterpreted the coding in setup_config()
as setting min_wal_size and max_wal_size to compile-time-constant
values.  But it's not: there's a hidden dependency on --wal-segsize.
Therefore leaving these variables commented out is the wrong thing.
Per report from Andres Freund.

Discussion: https://postgr.es/m/20230322200751.jvfvsuuhd3hgm6vv@awork3.anarazel.de