Drop dependency on gencpp target

[mikepurvis]

Not needed when using isolated build tools like catkin_tools and colcon, and throws this error:

--- stderr: yocs_cmd_vel_mux
CMake Warning (dev) at CMakeLists.txt:27 (add_dependencies):
  Policy CMP0046 is not set: Error on non-existent dependency in
  add_dependencies.  Run "cmake --help-policy CMP0046" for policy details.
  Use the cmake_policy command to set the policy and suppress this warning.

  The dependency target "geometry_msgs_gencpp" of target
  "yocs_cmd_vel_mux_nodelet" does not exist.
This warning is for project developers.  Use -Wno-dev to suppress it.

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