Remove unused methods

[thomshutt]

Confirm if IsExternal is used anywhere (i.e in other repos) and if so, document more clearly what its intended us is but otherwise remove.

Check other methods on the same interface.

go-tools/drivers/drivers.go

Line 126 in 44cbd85

IsExternal() bool

[thomshutt]

Looks like we do use it in a few places in go-livepeer, e.g https://github.com/livepeer/go-livepeer/blob/970d0aa7892cd9c30aaf069a7ee8e72d8fa64111/verification/epic.go#L191

so this task should just cover better documenting the intended usage