jflex AbstractLexScan: remove TODO

Char classes still need to be mutable at this point. They can only be
made immutable after the semantic analysis phase at the end of parsing.

jflex/src/main/java/jflex/core/AbstractLexScan.java

@@ -89,7 +89,6 @@ public UnicodeProperties getUnicodeProperties() {
     return unicodeProperties;
   }
 
-  // TODO(regisd) Return an immutable representation of char classes
   @SuppressWarnings("unused") // Used in generated LexParse
   public CharClasses getCharClasses() {
     return charClasses;