ext4: remove some redundant function declarations

ext4 update feature functions do not exist now, remove these useless
function declarations.
Signed-off-by: Shijie Luo <luoshijie1@huawei.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200724032954.22097-1-luoshijie1@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -2959,12 +2959,6 @@ do {									\
 
 #endif
 
-extern int ext4_update_compat_feature(handle_t *handle, struct super_block *sb,
-					__u32 compat);
-extern int ext4_update_rocompat_feature(handle_t *handle,
-					struct super_block *sb,	__u32 rocompat);
-extern int ext4_update_incompat_feature(handle_t *handle,
-					struct super_block *sb,	__u32 incompat);
 extern ext4_fsblk_t ext4_block_bitmap(struct super_block *sb,
 				      struct ext4_group_desc *bg);
 extern ext4_fsblk_t ext4_inode_bitmap(struct super_block *sb,