After https://github.com/tc39/ecma262/pull/642, Date.UTC no longer requires
the month argument to be specified. The spec provides 0 as its default value.

This CL updates the builtins-date.cc code to reflect that and drops the test
suppression for test262/built-ins/Date/UTC/return-value.

BUG=v8:5534

Review-Url: https://codereview.chromium.org/2689173003
Cr-Commit-Position: refs/heads/master@{#43193}