Code clean-up

See http://hg.openjdk.java.net/jdk/jdk/file/e6250a870739/src/hotspot/share/memory/allocation.hpp
http://hg.openjdk.java.net/jdk/jdk/file/e6250a870739/src/hotspot/share/services/nmtCommon.cpp

MemoryType and _memory_type_names should be in sync, but they are coded independently and it's possible to change one while forgetting to change the other.