Remove MagicSpells integration.

The integration only works reliably with the `clear-wave-` settings set
to `true`, because it relies on there being only one "current" wave.
This is a fundamental issue with the way sessions run right now, and is
another good example of why the Sessions Rework is necessary.

It is possible to achieve the exact same functionality by just making a
couple of Upgrade Waves before and after the waves that need to have
limited skill sets (thanks Lucy). With just a couple of quality-of-life
features like wave "triggers", it's possible to make things just the way
they were with the integration, but by using some more general building
blocks.

Closes #609

src/main/resources/magicspells.yml

@@ -1,18 +0,0 @@
-# MobArena disabled MagicSpells spells
-#
-# Use this file to disable the MagicSpells spells you don't want your players
-# to use during arena sessions. Make sure to put a hyphen (-) before every
-# spell, and try to keep the indentation proper.
-#
-# Spells that will be disabled for the entire arena session.
-disabled-spells:
-- carpet
-- blink
-
-# Spells that will be disabled only while a boss is present.
-disabled-on-bosses:
-- purge
-
-# Spells that will be disabled only during swarm waves.
-disabled-on-swarms:
-- purge