qpa: remove unused override for the inputcontext
Previously qtvirtualkeyboard was integrated weirdly so that it was acting as the focus object and proxy for input methods, however now that we support proper input methods, this code is totally unused in the QPA and actually prevents the QT_IM_MODULE from working now that qtvirtualkeyboard is dropped.
See: f26f2fe1 for the reference of the code deleted.