Rename onChangeHealth/Mana to onMana/HealthChange

The event name in creatures.xml also changed from:
changehealth and changemana to healthchange and manachange

If you were utilizing any of these creature events, you must update
their names for them to function.
Remove unreachable code

It was causing a warning when compiling in Visual Studio. If another
compiler for Windows complains, it may be added back with a preprocessor
conditional for that compiler.