pg_dump: Fix quoting of domain constraint names

The original code was adding double quotes to an already-quoted
identifier, leading to nonsensical results.  Remove the quoting call.

I introduced the broken code in 7eca575d of 9.5 era, so backpatch to
9.5.

Report and patch by Elvis Pranskevichus
Reviewed by Michael Paquier

src/bin/pg_dump/pg_dump.c

@@ -9611,7 +9611,7 @@ dumpDomain(Archive *fout, TypeInfo *tyinfo)
 		appendPQExpBuffer(labelq, "CONSTRAINT %s ",
 						  fmtId(domcheck->dobj.name));
 		appendPQExpBuffer(labelq, "ON DOMAIN %s",
-						  fmtId(qtypname));
+						  qtypname);
 		dumpComment(fout, labelq->data,
 					tyinfo->dobj.namespace->dobj.name,
 					tyinfo->rolname,