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/PlainDateTime/PlainDateTime.from.js

@@ -170,3 +170,12 @@ describe("errors", () => {
         }
     });
 });
+
+describe("errors", () => {
+    test("custom time zone doesn't have a getOffsetNanosecondsFor function", () => {
+        const zonedDateTime = new Temporal.ZonedDateTime(0n, {});
+        expect(() => {
+            Temporal.PlainDateTime.from(zonedDateTime);
+        }).toThrowWithMessage(TypeError, "null is not a function");
+    });
+});