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>