remove workaround for nested PreferenceScreen's

I know it's some minutes too late, but you always realize another thing when the changes are in... ;)

This workaround was introduced here: https://github.com/WhisperSystems/TextSecure/commit/815613cfaabdc456b86eb6fdccfcb61273d523f6

It fixed some theming issues with nested PreferenceScreen's. But we're not using them anymore (since a long time...), so I guess this is unnecessary.