Revert "Add support for block notifications using zeromq"

This reverts commit 15e3bbb098041e5aaaf077f125acf276040081fe.

	modified:   src/main.cpp
	modified:   src/makefile.unix
	modified:   src/net.cpp
	modified:   src/net.h
	deleted:    src/zeromq.cpp
	deleted:    src/zeromq.h