vk-driver: Don't listen for input source changes

Continuing from f5bb966b where we pushed listening to input-source changes from the virtual_keyboard to the vk_driver we can now remove it there too and just invoke pos_vk_driver_set_keymap().

This avoids listening for the same setting in two places and will also allow us to pass more information from the osk in use.

This will be useful in upcoming MRs.