Btrfs: retrurn void from clear_state_bit

Currently it returns a set of bits that were cleared, but this return
value is not used at all.

Moreover it doesn't seem to be useful, because we may clear the bits
of a few extent_states, but only the cleared bits of last one is
returned.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>