I fixed those warnings that were easy to fix: redundant imports, unused
variables, etc. (I might have missed some though)

The warnings I deliberately left for now are:
* In `Test.API.UpdateList`: Incomplete pattern matching (we would need
  to think a bit about error handling to fix this)
* In `Test.Utils`: `traceShowId` remains in code (we would need to think
  a bit about proper logging to fix this)