server: removed obsolete stat module

Knot.files

src/Makefile.am

src/knot/server/server.c

src/knot/server/udp-handler.c

src/knot/stat/gatherer.c

src/knot/stat/gatherer.h

src/knot/stat/stat-common.h

src/knot/stat/stat.c

src/knot/stat/stat.h