eis: after the client binds the seat, drop the capabilities

This way eis_seat_has_capability() returns the effective capabilities the server can actually use - no point creating touch devices when the client has not confirmed that.

In theory we should have a eis_seat_get_effective_capabilities() to differ between configured and effective capabilities, but I'm having a hard time thinking of a use-case where the implementation forgets which caps it enabled.

The side-effect of this patch is that adding a device without capabilities requested by the client now produces warning.

cc @davidre