Remove newly added useless assertion check

Remove newly added useless assertion check

Coverity complained that my commit 80ba4bb38353 added a dubious coding
for a consistency check that there isn't more than one row for a certain
tgrelid/tgparentid combination.  But we don't check for that explicitly
anywhere else, and if we were to do it, it should be a full
shouldn't-happen elog not just an assert.  It doesn't seem that this is
very important anyway, so remove it.

Discussion: https://postgr.es/m/1337562.1627224583@sss.pgh.pa.us