backends/x11: Fix access to WacomDevice
At some point we crossed the streams... In a short timespan we had 1f00aba9 merged, pushing WacomDevice to a common parent object, and dcaa45fc implementing device grouping for X11.
The latter did not rely on the former, and just happened to merge/compile without issues, but would promptly trigger a crash whenever the API would be used.
Drop all traces of the WacomDevice internal to MetaInputDeviceX11.