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/main/java/fr/xephi/authme/util/MigrationService.java

@@ -69,7 +69,7 @@ public final class MigrationService {
             try {
                 SQLite sqlite = new SQLite(settings);
                 ForceFlatToSqlite converter = new ForceFlatToSqlite(flatFile, sqlite);
-                converter.execute(null);
+                converter.run();
                 settings.setProperty(DatabaseSettings.BACKEND, DataSourceType.SQLITE);
                 settings.save();
                 return sqlite;