Retire src/backend/utils/misc/check_guc

This script has existed for a long time, and attempting to run it today
causes a lot of false positives as an effect of GUCs added in the last
couple of years.  An equivalent, automatically-run and cross-platform
solution is available in the TAP test introduced in b0a55f4.  So, let it
go.

Discussion: https://postgr.es/m/Yf9YGSwPiMu0c7fP@paquier.xyz

Author: michaelpq