Remove pin of YCM

[traversaro]

It should not be necessary after robotology/ycm-cmake-modules#458 .

[traversaro]

Argh, we now have again:

CMake Error at build/_deps/ycm-src/cmake-next/proposed/ExternalProject.cmake:3223 (_ep_parse_arguments):
  _ep_parse_arguments Function invoked with incorrect arguments for function
  named: _ep_parse_arguments
Call Stack (most recent call first):
  build/_deps/ycm-src/modules/YCMEPHelper.cmake:1113 (ExternalProject_Add)
  cmake/Buildhuman-gazebo.cmake:7 (ycm_ep_helper)
  build/_deps/ycm-src/modules/FindOrBuildPackage.cmake:205 (include)
  cmake/RobotologySuperbuildLogic.cmake:151 (find_or_build_package)
  CMakeLists.txt:67 (include)

In Ubuntu 20.04 GitHub Actions images. However, I think that is a particularly strange case as in that image there are multiple CMake installed with different versions. I think we can drop those CI tests, and just keep the one done in 20.04 with Docker.