Remove incorrect Assert.

check_agglevels_and_constraints() asserted that if we find an
aggregate function in an EXPR_KIND_FROM_SUBSELECT expression, the
expression must be in a LATERAL subquery.  Alexander Lakhin found a
case where that's not so: because of the odd scoping rules for NEW/OLD
within a rule, a reference to NEW/OLD could cause an aggregate to be
considered top-level even though it's in an unmarked sub-select.
The error message that would be thrown seems sufficiently on-point,
so just remove the Assert.  (Hence, this is not a bug for production
builds.)

This Assert was added by me in commit eaccfded9 (9.3 era).  It looks
like I put it in to cross-check that the new logic for detecting
misplaced aggregates (using agglevelsup) caught the same cases that a
previous check on p_lateral_active did.  So there might have been some
related misbehavior before eaccfded9 ... but that's very ancient
history by now, so I didn't dig any deeper.

Per bug #18608 from Alexander Lakhin.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/18608-48de0717508ee429@postgresql.org

src/backend/parser/parse_agg.c

@@ -374,8 +374,6 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 
 			break;
 		case EXPR_KIND_FROM_SUBSELECT:
-			/* Should only be possible in a LATERAL subquery */
-			Assert(pstate->p_lateral_active);
 
 			/*
 			 * Aggregate/grouping scope rules make it worth being explicit