Remove a FIXME related to recoveryTargetIsLatest. (#6863)

The recoveryTargetIsLatest setting code was missing somehow and later
it was added back in commit 55808e18. Removing the FIXME comment.
Reviewed-by: NJimmy Yih <jyih@pivotal.io>
Reviewed-by: NGeorgios Kokolatos <gkokolatos@pivotal.io>

src/backend/access/transam/xlog.c

@@ -292,7 +292,7 @@ static bool recoveryStopAfter;
  * to decrease.
  */
 static TimeLineID recoveryTargetTLI;
-static bool recoveryTargetIsLatest = false; // GPDB_93_MERGE_FIXME: should this be set somewhere?
+static bool recoveryTargetIsLatest = false;
 
 static List *expectedTLEs;
 static TimeLineID curFileTLI;