Now that just one method for stopping samples is left, let's just
repurpose st as the general "stop global effects" method rather than
have it there with a hyperspecific name. It also has good symmetry, as
there already was a `Start()` method in the class.
It is unnecessary, as a successful restart will exit the current player
screen, and `OnExiting()` has another `StopSampleAndRemoveFilters()`
call, which means that in the restart flow the sample was actually
getting stopped twice.
Standard exit flow is fine, it only stopped the sample once.