refactor: delete old combat code

[Jameskmonger]

The existing combat code had a few issues:

• the ticks were not synchronised between entities so hits weren't timed correctly
• tailing also wasn't synchronised between entities so the pathfinding would often put combatants in the wrong place
• the code was almost entirely within the Actor/NPC/Player classes

As we are going to be rewriting combat in the upcoming Task system, this PR removes the old code - we can use this PR as a reference when reimplementing it.

This PR also removes the Behaviour system as it was only used for combat and can be replaced with Tasks.

It also removes the Prayer plugin, as it is being replaced in the Task system, and the Effect system as they were only used by prayers.

[Jameskmonger]

@SchauweM changes addressed!