Monsters no longer load their script files

Before monsters loaded their script file on *each* spawn.
This is rather wrong since the script state keeps growing for each monster.
However all of this was unnessecary since the main.lua can simply require
the script file.

If you do not want to spam the main.lua (or any other file) with all the
requires: I guess we can add script binds to iterate over all monsters and
add some custom properties to the monsters.xml (See Mana-Mantis #356)