Remove misleading equality assertions `test_partial_string_to_time`

Calling `to_time` on a partial string returns a timestamp.

The exact value of this timestamp *cannot be asserted* unless we know whether the current time is "standard time" or "daylight savings time".

Rails code is aware of this, and in fact already includes:

* [one method](https://github.com/rails/rails/blob/810b7914f39ad6a6168ff19a8c2761601654f998/activesupport/test/core_ext/string_ext_test.rb#L488L508) to assert the value of the timestamp *when current time is standard time*, and
* [another method](https://github.com/rails/rails/blob/810b7914f39ad6a6168ff19a8c2761601654f998/activesupport/test/core_ext/string_ext_test.rb#L510L530) to assert the value of the timestamp *when current time is DST*.

Having [a separate test](https://github.com/rails/rails/blob/810b7914f39ad6a6168ff19a8c2761601654f998/activesupport/test/core_ext/string_ext_test.rb#L406L414) to assert the value *without specifying DST or not* is misleading, since it suggests that we can know the value independently of that.

Not that this separate test *has not failed so far* simply because it uses a *specific* time-zone "Europe/Moscow" that never switches from DST to standard time. For this and other few specific time zones, the test will still pass; however it will fail for any other time zone.

Therefore, I see two possible solutions:

1. Rename the assertions from `test_partial_string_to_time` to `test_partial_string_to_time_when_current_time_zone_does_not_use_daylight_savings`
1. Remove the assertions

This PR implements the last point (removing the assertions), since I don't think there is a lot of value in leaving a test that asserts values only based on specific preconditions (using a time zone rather than another).

I think that the other two methods linked above (`test_partial_string_to_time_when_current_time_is_standard_time` and `test_partial_string_to_time_when_current_time_is_daylight_savings`) already provide the coverage required by calling `String#to_time` with a partial string.