f2fs: kill EXT_TREE_VEC_SIZE

Since commit 201ef5e0 ("f2fs: improve shrink performance of extent nodes"),
there is no user of EXT_TREE_VEC_SIZE, just kill it for cleanup.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>