Retire MemoryContextResetAndDeleteChildren() macro.

As of commit eaa5808e8e, MemoryContextResetAndDeleteChildren() is
just a backwards compatibility macro for MemoryContextReset().  Now
that some time has passed, this macro seems more likely to create
confusion.

This commit removes the macro and replaces all remaining uses with
calls to MemoryContextReset().  Any third-party code that use this
macro will need to be adjusted to call MemoryContextReset()
instead.  Since the two have behaved the same way since v9.5, such
adjustments won't produce any behavior changes for all
currently-supported versions of PostgreSQL.

Reviewed-by: Amul Sul, Tom Lane, Alvaro Herrera, Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/20231113185950.GA1668018%40nathanxps13

src/backend/tcop/postgres.c

@@ -4457,7 +4457,7 @@ PostgresMain(const char *dbname, const char *username)
 		 * query input buffer in the cleared MessageContext.
 		 */
 		MemoryContextSwitchTo(MessageContext);
-		MemoryContextResetAndDeleteChildren(MessageContext);
+		MemoryContextReset(MessageContext);
 
 		initStringInfo(&input_message);