SQL/JSON: Remove useless code in ExecInitJsonExpr()

The code was for adding an unconditional JUMP to the next step,
which is unnecessary processing.

Reported-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxEo4sUjKCYtda0_qt9tazqqKPmF1cqhW9KBOUeJFqQd2g@mail.gmail.com
Backpatch-through: 17

src/backend/executor/execExpr.c

@@ -4312,21 +4312,6 @@ ExecInitJsonExpr(JsonExpr *jsexpr, ExprState *state,
 	scratch->d.constval.isnull = true;
 	ExprEvalPushStep(state, scratch);
 
-	/*
-	 * Jump to coerce the NULL using json_populate_type() if needed.  Coercing
-	 * NULL is only interesting when the RETURNING type is a domain whose
-	 * constraints must be checked.  jsexpr->use_json_coercion must have been
-	 * set in that case.
-	 */
-	if (get_typtype(jsexpr->returning->typid) == TYPTYPE_DOMAIN &&
-		DomainHasConstraints(jsexpr->returning->typid))
-	{
-		Assert(jsexpr->use_json_coercion);
-		scratch->opcode = EEOP_JUMP;
-		scratch->d.jump.jumpdone = state->steps_len + 1;
-		ExprEvalPushStep(state, scratch);
-	}
-
 	/*
 	 * To handle coercion errors softly, use the following ErrorSaveContext to
 	 * pass to ExecInitExprRec() when initializing the coercion expressions