[Temporal] Remove v8 staging tests plain-date-time-to-plain-*

One test in `test/staging/Temporal/v8/plain-date-time-to-plain-date.js` is covered by
`test/built-ins/Temporal/PlainDateTime/prototype/toPlainDate/branding.js`.
Move other one to `test/built-ins/Temporal/PlainDateTime/prototype/toPlainDate/basic.js`.

The tests in `test/staging/Temporal/v8/plain-date-time-to-plain-time.js` are covered by tests in
- `test/built-ins/Temporal/PlainDateTime/prototype/toPlainTime/basic.js`
- `test/built-ins/Temporal/PlainDateTime/prototype/toPlainTime/branding.js`

Author: ioannad

…oral/v8/plain-date-time-to-plain-date.js …nDateTime/prototype/toPlainDate/basic.jstest/staging/Temporal/v8/plain-date-time-to-plain-date.js renamed to test/built-ins/Temporal/PlainDateTime/prototype/toPlainDate/basic.js test/staging/Temporal/v8/plain-date-time-to-plain-date.js renamed to test/built-ins/Temporal/PlainDateTime/prototype/toPlainDate/basic.js

@@ -2,15 +2,11 @@
 // This code is governed by the BSD license found in the LICENSE file.
 
 /*---
-esid: pending
-description: >
-  Automatically ported from plain-date-time-to-plain-date test
-  in V8's mjsunit test plain-date-time-to-plain-date.js
+esid: sec-temporal.plaindatetime.prototype.withplaindate
+description: basic tests for toPlainDate
 includes: [temporalHelpers.js]
 features: [Temporal]
 ---*/
 
 let d1 = new Temporal.PlainDateTime(2021, 12, 11, 1, 2, 3, 4, 5, 6);
-let badDateTime = { toPlainDate: d1.toPlainDate };
-assert.throws(TypeError, () => badDateTime.toPlainDate());
 TemporalHelpers.assertPlainDate(d1.toPlainDate(), 2021, 12, 'M12', 11);