[Temporal] Remove v8 staging test plain-date-time-equals, -value-of, -to-json

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

Add `test/staging/Temporal/v8/plain-date-time-value-of.js` to
`test/built-ins/Temporal/PlainDateTime/prototype/valueOf/basic.js`.

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

Author: ioannad

test/built-ins/Temporal/PlainDateTime/prototype/valueOf/basic.js

@@ -11,6 +11,7 @@ const dt1 = new Temporal.PlainDateTime(1963, 2, 13, 9, 36, 29, 123, 456, 789);
 const dt1again = new Temporal.PlainDateTime(1963, 2, 13, 9, 36, 29, 123, 456, 789);
 const dt2 = new Temporal.PlainDateTime(1976, 11, 18, 15, 23, 30, 123, 456, 789);
 
+assert.throws(TypeError, () => dt1.valueOf(), "always throws");
 assert.sameValue(dt1 === dt1, true, "object equality implies ===");
 assert.sameValue(dt1 !== dt1again, true, "object non-equality, even if all data is the same, implies !==");
 assert.throws(TypeError, () => dt1 < dt1, "< throws (same objects)");