use UNUSED macro instead of casting to void

scripts/unused-macro.cocci

src/common/libtap/tap.c

src/common/mempattern.c

src/common/mempattern.h

src/common/prng.c

src/common/queue.c

src/common/slab/slab.c

src/knot/server/zones.c

src/libknot/sign/dnssec.c

src/libknot/updates/ddns.c