ceph: use generic_delete_inode() for ->drop_inode

ceph_drop_inode() implementation is not any different from the generic
function, thus there's no point in keeping it around.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/super.h

@@ -886,7 +886,6 @@ extern const struct inode_operations ceph_file_iops;
 extern struct inode *ceph_alloc_inode(struct super_block *sb);
 extern void ceph_evict_inode(struct inode *inode);
 extern void ceph_free_inode(struct inode *inode);
-extern int ceph_drop_inode(struct inode *inode);
 
 extern struct inode *ceph_get_inode(struct super_block *sb,
 				    struct ceph_vino vino);