package/dovecot: Remove bzip2 and zlib options

The next patch of this series will add optional xz and lz4 support, to
avoid adding new options for these compression packages simplify the
configuration of dovecot by removing the options handling optional
compression support.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>