remove dead playWhenSkipped variable in renderListTasks

[josegonzalez]

Description

renderListTasks declares playWhenSkipped, assigns it, and then discards it:

playWhenSkipped := false
if play.HasWhen() {
    // ...
    if !ok {
        playWhenSkipped = true
        // ... emits the skip line, then:
        continue
    }
}
_ = playWhenSkipped

Every path that sets the variable continues before it is read, so the _ = playWhenSkipped
discard at the end is the only thing keeping the compiler quiet. The declaration, the assignment,
and the discard can all go.

Noticed while working on #412, which moved the play-scoping bats tests onto plan --list-tasks.
Left out of that PR to keep it a test-only change.

Location

commands/list_tasks.go:110, :128, :142