Revert change to is_local_id as it's not needed

Upon closer inspection, the `isIdentifierChar` in the RubyLexer class
can correctly handle non-ascii characters fine, so it doesn't matter
if we pass in just the first byte of a multi-byte encoded char, we'll
still get the same result.

PullRequest: truffleruby/2102

Author: ivoanjo

src/main/java/org/truffleruby/parser/parser/ParserSupport.java

@@ -1459,7 +1459,7 @@ public void warning(SourceIndexLength position, String message) {
 
     // ENEBO: Totally weird naming (in MRI is not allocated and is a local var name) [1.9]
     public boolean is_local_id(Rope name) {
-        return lexer.isIdentifierChar(name.getString().charAt(0));
+        return lexer.isIdentifierChar(name.get(0) & 0xFF);
     }
 
     // 1.9