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.