refactor(mock_command): Remove print statement

[TheJanzap]

Remove the unnecessary print statement on MockCommand.run(), as I don't think it is really necessary. Plus, it breaks output on flutter test --reporter github:

✅ /builds/foo/app/test/foo_test.dart: Working test 1
::group::✅ /builds/foo/app/test/foo_test.dart: Broken test with MockCommand.run
Called Execute
::endgroup::
✅ /builds/foo/app/test/foo_test.dart: Working test 2