Remove incorrect isRoleMirror assertion for FTS_PROMOTE_SUCCESS.

Incase promote message is send twice while mirror already undergoing promotion,
isRoleMirror can be retruned as false. ON mirror in this case promote message is
ignored. So, on master fts shouldn't check what the isRoleMirror returns.

src/backend/fts/ftsprobe.c

@@ -1139,7 +1139,6 @@ processResponse(fts_context *context)
 				ftsInfo->state = FTS_RESPONSE_PROCESSED;
 				break;
 			case FTS_PROMOTE_SUCCESS:
-				Assert(ftsInfo->result.isRoleMirror);
 				elogif(gp_log_fts >= GPVARS_VERBOSITY_VERBOSE, LOG,
 					   "FTS mirror (content=%d, dbid=%d) promotion "
 					   "triggerred successfully",