Fix titles for contextual filters

When a contextual filter is set, we currently return 'Search @field for "@arg"' as the title. This stemmed from too little understanding on my part when writing this integration, not taking into account that users can easily apply such formatting themselves and that therefore only the argument should be returned.

We should therefore remove our own title() implementations as long as we have nothing to offer in terms of formatting the argument itself in a more meaningful way (which we don't).
This also makes the SearchApiViewsHandlerArgumentText class unnecessary, which currently only contains that single method.