Remove test that fails because of DST

Problem
-------

The [following test](https://github.com/rails/rails/blob/d20f7b043a537b57ff4a7911f65de2fb7b7aea7d/activesupport/test/core_ext/string_ext_test.rb#L411) fails on my machine

```ruby
StringConversionsTest#test_partial_string_to_time [test/core_ext/string_ext_test.rb:411]:
Expected: 2014-10-30 18:50:00 +0300
  Actual: 2014-10-30 17:50:00 +0300
```

Cause
-----

In the United States, Daylight Saving Time ends on the first Sunday of November (see [Wikipedia article](http://en.wikipedia.org/wiki/Daylight_saving_time_by_country#Daylight_Saving_Time_rules_as_of_2014)).
In Russia, Daylight Saving Time is… undergoing changes (as pointed out in #17395 and #17409).

Since I am running the tests from a machine located in the United States (PST), the time difference expected by the test does not always hold true.
The same test might be passing when run from a machine in a different time zone.

Solution
--------

I think it's safe to simply remove the line of failing test.

`Time.local` and `.to_time` are already been tested in the other lines of the same test file.

The failing test is based on the presumption that "18.50 local time in Moscow" equals "13:50 -0100", and when that does not hold true, the test fails.

How to reproduce
----------------

1. Temporarily change the date of your machine to October 30th, 2014 (to ensure DST has not ended)
2. Set the time zone of your machine to PST
3. Run `ruby -Itest test/core_ext/string_ext_test.rb`