Remove no longer needed String#to_datetime patch.

Fixes #826

From: https://bugzilla.redhat.com/show_bug.cgi?id=MANAGEIQ-5898

```
The root cause was that under Rails 2.2 SOAP4r calls a "to_datetime" method on
a string which is a method added by ActiveSupport.  The method was ignoring the
timezone and caused the VC time to be inaccurate.
```

With the patch:

```
irb(main):001:0> require 'patches/active_support_string_patch'
=> false
irb(main):002:0> Time.now.to_s.to_datetime.to_s
=> "2014-10-14T18:04:13-04:00
```

Without the patch:
**Note**: the timezone is no longer ignored, so the patch can be removed:

```
irb(main):001:0> Time.now.to_s.to_datetime.to_s
=> "2014-10-14T18:03:38-04:00"
```