ecpg: remove check_rules.pl.
As noted in the previous commit, check_rules.pl is now entirely
redundant with checks made by parse.pl, or would be if it weren't
for the places where it's wrong. It's a waste of build cycles
and maintenance effort, so remove it.
Discussion: https://postgr.es/m/
2011420.
1713493114@sss.pgh.pa.us