Remove GPDB_12_MERGE_FIXME in ic_tcp.c (#17154)

GPDB_12_MERGE_FIXME in ic_tcp.c:
should use WaitEventSetWait() instead of select() following the routine in ic_udpifc.c

The advantage of using WaitEventSetWait instead of select is that we can monitor
the specific waitevent in the pg_stat_activity view. 

However, we still have many select() calls in current Greenplum codes, it's no need to
replace all of them with WaitEventSet, Because most of them doesn't benefit much
from pg_stat_activity, keep using select() is good and simple enough.
Therefore, we have decided to simply remove this fixme.

src/backend/cdb/motion/ic_tcp.c

@@ -2655,8 +2655,6 @@ RecvTupleChunkFromAnyTCP(ChunkTransportState *transportStates,
 
 		}
 
-		// GPDB_12_MERGE_FIXME: should use WaitEventSetWait() instead of select()
-		// follow the routine in ic_udpifc.c
 		n = select(nfds + 1, (fd_set *) &rset, NULL, NULL, &timeout);
 		if (n < 0)
 		{