Fix assertion failure for REFRESH MATERIALIZED VIEW in PL.

This was due to incomplete implementation of rowcount reporting
for RMV, which was due to initial waffling on whether it should
be provided.  It seems unlikely to be a useful or universally
available  number as more sophisticated techniques for maintaining
matviews are added, so remove the partial support rather than
completing it.

Per report of Jeevan Chalke, but with a different fix

src/backend/executor/spi.c

@@ -2122,13 +2122,6 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
 					if (((CreateTableAsStmt *) stmt)->is_select_into)
 						res = SPI_OK_SELINTO;
 				}
-				else if (IsA(stmt, RefreshMatViewStmt))
-				{
-					Assert(strncmp(completionTag,
-								   "REFRESH MATERIALIZED VIEW ", 23) == 0);
-					_SPI_current->processed = strtoul(completionTag + 23,
-													  NULL, 10);
-				}
 				else if (IsA(stmt, CopyStmt))
 				{
 					Assert(strncmp(completionTag, "COPY ", 5) == 0);