Code householding (PlayerListener / Settings)

- Use field on PlayerListener for storing nickname pattern -> repeatedly parsing pattern is expensive
- Remove unused legacy setting fields
- ForceFlatToSqlite cannot be run from converter command -> remove Converter interface to create more natural method signatures

src/test/java/fr/xephi/authme/converter/ForceFlatToSqliteTest.java

@@ -54,7 +54,7 @@ public class ForceFlatToSqliteTest {
         ForceFlatToSqlite converter = new ForceFlatToSqlite(flatFile, dataSource);
 
         // when
-        converter.execute(null);
+        converter.run();
 
         // then
         ArgumentCaptor<PlayerAuth> authCaptor = ArgumentCaptor.forClass(PlayerAuth.class);