They are no longer used, and cannot be efficiently implemenented
for large bitsets using a summary vector approach without slowing
down the find_*_set() variants, which are used.
Also remove find_previous_set() for the same reason.
A dynamic bitset implementation that provides functions to search for
both set and cleared bits in both directions.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
