Remove Windows CI Jenkins support for Gazebo-Classic

[j-rivero]

Current Windows builds are broken due to #3327 on Jenkins.

The CI support for Windows on Jenkins has been quite ugly since the beginning with the use of ancient tools (like jom) and the lack of a real and good dependency system.

Starting with #3186 the great @traversaro added the conda-forge Windows CI build which is a much better alternative and probably the best way that Gazebo Classic has for Windows nowadays.

The development team agreed in todays Gazebo meeting to remove the Windows CI test on Jenkins for Gazebo Classic. If someone don't think that this is a god idea, please speak now.

fix #3327