Remove most iron test_msgs (#18)

### Changelog
Remove most iron `test_msgs` messages

### Docs
None

### Description
#14 added additional `test_msgs` message definitions which caused
issues, especially due to `wstring` usage in `test_msgs/WStrings`. This
PR removes these additional message files again, leaving just
`test_msgs/Builtins.msg` which is also included in galactic and humble.

Author: achim-k