LibJS: Remove fallback value for get_offset_nanoseconds_for

This is a normative change in the Temporal spec.

See: 664f02d

Note that the tests are not comprehensive.

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDateTime.js

@@ -37,3 +37,11 @@ describe("correct behavior", () => {
         expect(Math.floor(differenceSeconds)).toBe(86400);
     });
 });
+
+describe("errors", () => {
+    test("custom time zone doesn't have a getOffsetNanosecondsFor function", () => {
+        expect(() => {
+            Temporal.Now.plainDateTime({}, {});
+        }).toThrowWithMessage(TypeError, "null is not a function");
+    });
+});