force-leave does not remove the failed node from the raft voters list

#### Overview of the Issue

`force-leave` command does not remove the failed node from the list of raft voters in a case when the quorum is lost. From another side, the graceful `leave` in a similar scenario does not cause the loss of the quorum in the first place and works fine.

#### Reproduction Steps

1. Create a cluster with 2 server nodes and bootstrap it with `-bootstrap-expect=1`.
1. Crash 1 of the nodes.
1. Run `consul force-leave ` on the alive node.

It seems natural to expect that `force-leave` would remove the failed node from the raft voters and the quorum would be restored (like in the case with graceful [leave](https://www.consul.io/docs/commands/leave.html)), but it does not and the cluster effectively remains stuck.

(Tested on 1.5.3, 1.6.1 and 1.6.2 versions.)