btrfs: remove redundant warning message in btrfs_check_uuid_tree()

btrfs: remove redundant warning message in btrfs_check_uuid_tree()

If we fail to start the UUID rescan kthread, btrfs_check_uuid_tree() logs
an error message and returns the error to the single caller, open_ctree().

This however is redundant since the caller already logs an error message,
which is also more informative since it logs the error code. Some remove
the warning message from btrfs_check_uuid_tree() as it doesn't add any
value.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>