* the `common.destroy` function is odd
  * accepts a nullable channel (instead of just not being called)
  * has a funny return signature
  * the fact that it is a method on a channel rather than a function
    call that accepts a channel (or phone number) forces some tightly
    coupled testing patterns (that could be cleaner with pure functions
    on the db) without saving any db queries by doing so
* replace it with `channelRepository.destroy` and also create
  `phoneNumberRepository.destroy`: leaves cleaner test seams and less
  convoluted branching logcis
* clean up tests for destroy that (1) break b/c of this, (2) were
  tightly coupled and a bit hard to follow/change anyway