Remove obsolete ServerMocks after master merge

The merge from master reintroduced ServerMocks.java and added a
stale import to MobsSpawnListenerTest. ServerMocks predates the
MockBukkit-based test infrastructure and no longer compiles against
Paper 1.21.11 (its Registry.get call is ambiguous against the new
Registry<TypedKey<T>> overload). MockBukkit covers the same ground
via CommonTestSetup, so drop the file and its now-unused import.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tastybento

src/test/java/world/bentobox/extramobs/listeners/MobsSpawnListenerTest.java

@@ -32,7 +32,6 @@
 import world.bentobox.extramobs.ExtraMobsAddon;
 import world.bentobox.extramobs.config.MobSpawnReplacement;
 import world.bentobox.extramobs.config.Settings;
-import world.bentobox.extramobs.listeners.mocks.ServerMocks;
 
 /**
  * Tests for {@link MobsSpawnListener}.