make StepDownOnRemoval the default behavior

[erikgrinaker]

In #79, we added StepDownOnRemoval, which makes a leader step down to follower after removing itself from the group or demoting itself to learner. Without this, the old leader may keep heartbeating even though it's no longer part of the group, and it can't propose anything.

However, for backwards compatibility reasons, this behavior is off by default. In 4.0, we should always do this and remove the setting.