Disable "cannot make new WAL entries during recovery" check again.

The check was introduced by the 8.4 merge, but I had disabled it because
we were tripping it. I re-enabled it in commit 1df4698a, thinking that
the other changes in that commit made it work again, but we're seeing
pipeline failures on many filerep-related test suites because of this.
So disable it again.

We really shouldn't hit that sanity check, but the current plan is to
revamp this code greatly before the next release, as we are about to start
replacing the GPDB-specific file replication with upstream WAL replication.
Chances are that it will start to just work once that work is done, so I'm
not going to spend any more time investigating this right now.

Analysis by Jacob Champion.

src/backend/access/transam/xlog.c

@@ -907,9 +907,13 @@ XLogInsert_Internal(RmgrId rmid, uint8 info, XLogRecData *rdata, TransactionId h
 		}
  	}
 
+	/* GPDB_84_MERGE_FIXME: This cross-check was added in upstream, but it's failing
+	 * in Startup pass 2. Disable it for now. */
+#if 0
 	/* cross-check on whether we should be here or not */
 	if (!XLogInsertAllowed())
 		elog(ERROR, "cannot make new WAL entries during recovery");
+#endif
 
 	/* info's high bits are reserved for use by me */
 	if (info & XLR_INFO_MASK)