remove support for const (#1910)

As this is not part of ES5.

test/mocha/cli.js

@@ -379,21 +379,6 @@ describe("bin/uglifyjs", function () {
            done();
        });
     });
-    it("Should throw syntax error (const a)", function(done) {
-       var command = uglifyjscmd + ' test/input/invalid/const.js';
-
-       exec(command, function (err, stdout, stderr) {
-           assert.ok(err);
-           assert.strictEqual(stdout, "");
-           assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
-               "Parse error at test/input/invalid/const.js:7,11",
-               "    const a;",
-               "           ^",
-               "ERROR: Missing initializer in const declaration"
-           ].join("\n"));
-           done();
-       });
-    });
     it("Should throw syntax error (delete x)", function(done) {
        var command = uglifyjscmd + ' test/input/invalid/delete.js';