Remove the build_depend.

It is not necessary to have rosidl_generator_c as a build_depend
for this package, just as an export (and for tests).

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

rosidl_generator_cpp/package.xml

@@ -7,8 +7,6 @@
   <maintainer email="dthomas@osrfoundation.org">Dirk Thomas</maintainer>
   <license>Apache License 2.0</license>
 
-  <build_depend>rosidl_generator_c</build_depend>
-
   <buildtool_depend>ament_cmake</buildtool_depend>
 
   <buildtool_export_depend>ament_cmake</buildtool_export_depend>