control: Drop Depends on libxt6

We don't actually link against it, we just intercept one of its functions via libswmhack. If the library is not present on the system, by definition none of the applications that use it will be able to start, and there will be nothing to intercept. So a dependency on our side is not useful, and it just gets in the way of the ongoing 64-bit time_t transition.

Closes: #1068203