Make ament_cmake a buildtool dependency (ros2#2689)

* Make ament_cmake a buildtool dependency

The `ament_cmake` package isn't needed at runtime and so should only be
listed as a `buildtool_dependency`, as is done in most other packages.

* Remove the ament_cmake dependency entirely

Since there's already a buildtool dependency on ament_cmake_ros, having
ament_cmake as well is redundant.

Signed-off-by: Nathan Wiebe Neufeldt <nwiebeneufeldt@clearpath.ai>

rclcpp_action/package.xml

@@ -26,8 +26,6 @@
   <depend>rcl</depend>
   <depend>rcpputils</depend>
 
-  <depend>ament_cmake</depend>
-
   <test_depend>ament_cmake_gtest</test_depend>
   <test_depend>ament_lint_auto</test_depend>
   <test_depend>ament_lint_common</test_depend>