Discovered in #925

Tests run each time we change something in the code while mix phx.server is running. This is not really our work flow and hasn't been like this since docker times, so this PR simply removes the mix_test_watch dependency.

To explain why this suddenly started happening.

We had it as a dependency, but in the move to Phoenix 1.3, I also performed a migration to Elixir 1.4

One of the new features of 1.4 is that we no longer need to explicitly specify an applications: [] list parameter, to specify which applications need to run alongside the main application. Instead, this is inferred from the dependency list.

Prior to the move, we did specify a list explicitly, and mix_test_watch wasn't in it, so it didn't run. Basically, we had an external dependency which we weren't using.

In the migration PR, I removed this list, so now it's being inferred, so now mix_test_watch runs. Since this was an unused dependency before the move, the "fix" is to explicitly remove it.