Removed protections against user code redefining undefined

Jasmine hasn't even run on platforms that allowed redefining undefined
since 2.x.

src/core/MockDate.js

@@ -15,7 +15,7 @@ getJasmineRequireObj().MockDate = function(j$) {
       if (mockDate instanceof GlobalDate) {
         currentTime = mockDate.getTime();
       } else {
-        if (!j$.util.isUndefined(mockDate)) {
+        if (mockDate !== undefined) {
           throw new Error(
             'The argument to jasmine.clock().mockDate(), if specified, ' +
               'should be a Date instance.'