Remove Btrfs compat code for older kernels

Btrfs had compatibility code for kernels back to 2.6.18.  These have
been removed, and will be maintained in a separate backport
git tree from now on.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/sysfs.c

@@ -28,7 +28,6 @@
 #include "disk-io.h"
 #include "transaction.h"
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,25)
 static ssize_t root_blocks_used_show(struct btrfs_root *root, char *buf)
 {
 	return snprintf(buf, PAGE_SIZE, "%llu\n",
@@ -267,35 +266,3 @@ void btrfs_exit_sysfs(void)
 	kset_unregister(btrfs_kset);
 }
 
-#else
-
-int btrfs_sysfs_add_super(struct btrfs_fs_info *fs)
-{
-	return 0;
-}
-
-int btrfs_sysfs_add_root(struct btrfs_root *root)
-{
-	return 0;
-}
-
-void btrfs_sysfs_del_root(struct btrfs_root *root)
-{
-	return;
-}
-
-void btrfs_sysfs_del_super(struct btrfs_fs_info *fs)
-{
-	return;
-}
-
-int btrfs_init_sysfs(void)
-{
-	return 0;
-}
-
-void btrfs_exit_sysfs(void)
-{
-	return;
-}
-#endif