Remove nonsense #undef foreach

This commit removes a preprocessor #undef for a macro that was never
defined. Should make no difference to compiled code.

src/txmempool.h

@@ -20,7 +20,6 @@
 #include "sync.h"
 #include "random.h"
 
-#undef foreach
 #include "boost/multi_index_container.hpp"
 #include "boost/multi_index/ordered_index.hpp"
 #include "boost/multi_index/hashed_index.hpp"