Remove warn/2 function

It's better just to call error_logger:warning_msg/2 here.
See also commit f9a10697bcf254c6568e49f0dc3ddaed5c9caae5

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Merge branch 'drop_warn' of https://github.com/lemenkov/bitcask into develop