PEG.js grammar: Remove reserved word detection

This is mostly done for consistency with the JavaScript example grammar,
from which the |Identifier| rule is taken from. See the previous commit
for details.

spec/parser.spec.js

@@ -388,10 +388,6 @@ describe("PEG.js grammar parser", function() {
   /* Canonical Identifier is "a". */
   it("parses Identifier", function() {
     expect('start = a:"abcd"').toParseAs(oneRuleGrammar(labeledAbcd));
-
-    expect('start = return:"abcd"').toFailToParse({
-      message: "Reserved word \"return\" can't be used as an identifier."
-    });
   });
 
   /* Canonical IdentifierName is "a". */

src/parser.pegjs

@@ -218,9 +218,6 @@ SingleLineComment
 
 Identifier
   = !ReservedWord name:IdentifierName { return name; }
-  / name:IdentifierName {
-      error("Reserved word \"" + name + "\" can't be used as an identifier.");
-    }
 
 IdentifierName "identifier"
   = first:IdentifierStart rest:IdentifierPart* { return first + rest.join(""); }