Some spring cleaning

2 code related "cleanings" are:
 - remove `SSHKit::Command` class dependency on `shellwords` std lib. It seems `sshkit` doesn't use that lib since e73c57a. I checked and `capistrano` also does not use it.
 - remove `SSHKit::Coordinator#cooldown` private accessor. This line was introduced in b71fba1ee5d9ca25a4741de954bddc34c391e505. From what I see, it hasn't been used anywhere really.

Other than that, this pull req mostly removes trailing whitespace from files.