Remove obsoleted code relating to targetlist SRF evaluation.

Since 69f4b9c plain expression evaluation (and thus normal projection)
can't return sets of tuples anymore. Thus remove code dealing with
that possibility.

This will require adjustments in external code using
ExecEvalExpr()/ExecProject() - that should neither be hard nor very
common.

Author: Andres Freund and Tom Lane
Discussion: https://postgr.es/m/20160822214023.aaxz5l4igypowyri@alap3.anarazel.de

src/backend/executor/nodeProjectSet.c

@@ -169,7 +169,7 @@ ExecProjectSRF(ProjectSetState *node, bool continuing)
 		else
 		{
 			/* Non-SRF tlist expression, just evaluate normally. */
-			*result = ExecEvalExpr(gstate->arg, econtext, isnull, NULL);
+			*result = ExecEvalExpr(gstate->arg, econtext, isnull);
 			*isdone = ExprSingleResult;
 		}