LibJS: Remove redundant ToString from ToTemporalZonedDateTime

Assert that the type of offsetString is either undefined or
string in order to avoid unnecessary ToString
Temporal commit: tc39/proposal-temporal@24ebcbd

Userland/Libraries/LibJS/Runtime/Temporal/ZonedDateTime.cpp

@@ -167,16 +167,14 @@ ThrowCompletionOr<ZonedDateTime*> to_temporal_zoned_date_time(VM& vm, Value item
         // i. Let offsetString be ! Get(fields, "offset").
         auto offset_string_value = MUST(fields->get(vm.names.offset));
 
-        // j. If offsetString is undefined, then
+        // j. Assert: offsetString is a String or undefined.
+        VERIFY(offset_string_value.is_string() || offset_string_value.is_undefined());
+
+        // k. If offsetString is undefined, then
         if (offset_string_value.is_undefined()) {
             // i. Set offsetBehaviour to wall.
             offset_behavior = OffsetBehavior::Wall;
         }
-        // k. Else,
-        else {
-            // i. Set offsetString to ? ToString(offsetString).
-            offset_string = TRY(offset_string_value.to_deprecated_string(vm));
-        }
 
         // l. Let result be ? InterpretTemporalDateTimeFields(calendar, fields, options).
         result = TRY(interpret_temporal_date_time_fields(vm, *calendar, *fields, *options));