Remove raise on specification removal

The gemspec is removed from the filesystem prior to removing from the
Specification list, so there is an order dependence here.

This causes issues in vagrant:

https://github.com/mitchellh/vagrant/pull/2420/files#diff-f6bf52fb33f7f429df40149aa99c126dR131

and can be reproduced by the following code:

```
require 'rubygems'
require 'rubygems/uninstaller'

name = 'vcr'
version = '2.6.0'
`gem install #{name} -v #{version}`

Gem::Uninstaller.new(name, {
  :all         => true,
  :executables => true,
  :force       => true,
  :ignore      => true,
  :version     => version
}).uninstall
```