initdb: Do not report default autovacuum_worker_slots.

initdb: Do not report default autovacuum_worker_slots.

Commit 6d01541960 taught initdb to lower the default value of
autovacuum_worker_slots for systems with very few semaphores.  It
also added a "fake" report for the chosen value, i.e., initdb
prints a message about selecting the default, but the value was
already selected in a previous test.  Per discussion, this is not a
precedent we want to set, and it seems unnecessary to report
everything derived from max_connections, so let's remove the "fake"
report.

Reported-by: Peter Eisentraut 
Suggested-by: Tom Lane 
Discussion: https://postgr.es/m/de722583-4ba4-4063-bc41-e20684978116%40eisentraut.org