make `StepDownOnRemoval` the default behavior

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.

Start by locating the StepDownOnRemoval implementation and the setting's callers, then inspect the behavior added in #79 for removal and demotion to learner. Remove the setting and verify that both paths always make the leader step down, using any existing tests around those operations as the completion check.

Written by the indexing model from the issue text.