fix incorrect clearing of bits in bitarray

fix incorrect clearing of bits in bitarray

The wrong bitwise operations were used here, resulting in e.g. a set of bit 2 of
of `0b011000` updating to `0b011100` instead of being a no-op.

This bug was present since the first introduction of this interface in
3ced90bd756bb8e56920620f2686e0ba6b482bbb, but this has not yet made it into a
release.