[kcm/touchpad] Drop synaptics config

The synaptics touchpad driver has mostly been superseeded by libinput.

For libinput we have a completely different UI.

The KCM currently still supports both, making the code massively complex and
causing a huge maintanance burden.

Given the long-term focus is on Wayland and libinput drop the synaptic integration.

This doesn't prevent anyone from using Plasma X11 with Synaptics, but there will
not be a first-party config UI for it any more

See #79