btrfs: remove btrfs_err_str function from uapi/linux/btrfs.h

btrfs_err_str function is not called from anywhere and is replicated
in the userspace headers for btrfs-progs.

It's removal also fixes the following linux/btrfs.h userspace
compilation error:

/usr/include/linux/btrfs.h: In function 'btrfs_err_str':
/usr/include/linux/btrfs.h:740:11: error: 'NULL' undeclared (first use in this function)
    return NULL;
Suggested-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs.h

@@ -713,33 +713,6 @@ enum btrfs_err_code {
 	BTRFS_ERROR_DEV_ONLY_WRITABLE,
 	BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS
 };
-/* An error code to error string mapping for the kernel
-*  error codes
-*/
-static inline char *btrfs_err_str(enum btrfs_err_code err_code)
-{
-	switch (err_code) {
-		case BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET:
-			return "unable to go below two devices on raid1";
-		case BTRFS_ERROR_DEV_RAID10_MIN_NOT_MET:
-			return "unable to go below four devices on raid10";
-		case BTRFS_ERROR_DEV_RAID5_MIN_NOT_MET:
-			return "unable to go below two devices on raid5";
-		case BTRFS_ERROR_DEV_RAID6_MIN_NOT_MET:
-			return "unable to go below three devices on raid6";
-		case BTRFS_ERROR_DEV_TGT_REPLACE:
-			return "unable to remove the dev_replace target dev";
-		case BTRFS_ERROR_DEV_MISSING_NOT_FOUND:
-			return "no missing devices found to remove";
-		case BTRFS_ERROR_DEV_ONLY_WRITABLE:
-			return "unable to remove the only writeable device";
-		case BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS:
-			return "add/delete/balance/replace/resize operation "\
-				"in progress";
-		default:
-			return NULL;
-	}
-}
 
 #define BTRFS_IOC_SNAP_CREATE _IOW(BTRFS_IOCTL_MAGIC, 1, \
 				   struct btrfs_ioctl_vol_args)