f2fs: remove unused FORCE_FG_GC macro

FORCE_FG_GC was introduced by commit 6aefd93b ("f2fs: introduce
background_gc=sync mount option"), but never be used, remove it.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -172,12 +172,10 @@ enum {
 /*
  * BG_GC means the background cleaning job.
  * FG_GC means the on-demand cleaning job.
- * FORCE_FG_GC means on-demand cleaning job in background.
  */
 enum {
 	BG_GC = 0,
 	FG_GC,
-	FORCE_FG_GC,
 };
 
 /* for a function parameter to select a victim segment */