ext4: remove unused function prototype from ext4.h

The ext4_extent_tree_init() function hasn't been in the ext4 code for
a long time ago, except in an unused function prototype in ext4.h

Google-Bug-Id: 4530137
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -2889,7 +2889,6 @@ extern int ext4_map_blocks(handle_t *handle, struct inode *inode,
 			   struct ext4_map_blocks *map, int flags);
 extern int ext4_ext_calc_metadata_amount(struct inode *inode,
 					 ext4_lblk_t lblocks);
-extern int ext4_extent_tree_init(handle_t *, struct inode *);
 extern int ext4_ext_calc_credits_for_single_extent(struct inode *inode,
 						   int num,
 						   struct ext4_ext_path *path);