Remove permanent and code cache from heap graph

They're not part of the Java Heap and will be made visible again in the
total JVM memory usage graph.
Still too much false negatives here...

The memory block is 131584 kB
statistics shows committed 106496 kB and max 117248 kB

So, the max reported from inside the JVM is not really the max as set
with -Xmx.... It would be better to be able to access the Xmx setting in
this plugin. Sadly, this setting has always been used in the list of jvm
parameters, instead of a separate memory setting that could be used for
Xmx and Xms. Option 1: parse it back from the jvmopts list... Option 2:
change m2ee-tools to use a memory option instead. Creating option 2
would still not eliminate the need to have option 1 backwards compatible
for a long time......
Prevent graph from disappearing when app not running

OTOH, this will show a graph without contents when running on a system
that does not have /proc/pid/smaps... or when running munin without the
patch that properly sets group permissions for plugins...
JVM memory graph: show Java Heap internals and PermGen

This makes the graph even more fun and educational!