Remove unused AMENT_CMAKE_ENVIRONMENT_GENERATION option

[sloretz]

This removes a CMake option AMENT_CMAKE_ENVIRONMENT_GENERATION that does not appear to be used anywhere, and is using scripts that I can only find uses of in the abandoned ament_tools project.

I came across this while looking at upgrading ament_cmake to use FindPython3 instead of FindPythonInterp. These "prefix_level" scripts reference PYTHON_EXECUTABLE from FindPythonInterp, but as far as I can tell they're not used anywhere, and haven't been since ament_tools.

Part of ros2/python_cmake_module#6
Part of ament/ament_package#133

[sloretz]

CI lgtm

[nuclearsandwich]

I'm not sure how much we need to consider "legacy" but I'll share what I recollect about this script option.

All catkin packages generate prefix level environment scripts by default unless CATKIN_BUILD_BINARY_PACKAGE is true or CATKIN_INSTALL_INTO_PREFIX_ROOT is set to false.

In ament the environment generation is off by default and, as you mention, colcon currently provides its own implementation of the prefix-level environment scripts for our recommended build-from-source workflow.

However, the prefix-level scripts for binary packages are provided by the ros_workspace package which calls ament_generate_environment() from ament_cmake_core directly thus skipping the AMENT_CMAKE_ENVIRONMENT_GENERATION option.

With all that in mind, I'm not sure whether the option provides any value but we have to keep the rest of the content around for the ros_workspace package.

[sloretz]

@nuclearsandwich mind taking a second look? I've updated this PR to only remove the CMake option, and the other PR to only remove the isolated_prefix_level scripts which still seem to be unused.

Added back still used code