remove duplicate field stripping

@bf4 @jeremy @mikel 

calling afield with it's own name seems like a pretty useless feature, removing this breaks ~30 specs, that I'd fix if we decide to go this way ...

https://github.com/mikel/mail/pull/720

fix:
```Ruby
# https://github.com/mikel/mail/pull/766
Mail::CommonField.class_eval do
  def strip_field(field_name, value)
    value
  end
end
```
Example (where you probably saw some failing specs, I assume) of expecting a string `value`.