Remove python3 dependencies implied by ament_python build type. (#53)
Remove python3 dependencies implied by ament_python build type. (#53)

From https://github.com/ros/rosdistro/pull/15328 there is no longer a rosdep key for `python3`.
All ament_python packages have injected build_depends on Debian packages python3-all, dh-python, and python3-setuptools.
The Debian package builder also adds python runtime dependencies as it sees fit via the `${python3:Depends}` helper.

I think there's a long term danger of too-closely relying on assumed dependencies, particularly for platform-specific builds, but for consistency with other ament_python packages I have removed these here.

If we want to leave these dependencies untouched in the source package.xml, they'll need to be patched out at release time since there is no longer a rosdep key `python3`.
ament/ament_package,ament/ament_package