OGS5 executable downloads broken

[MuellerSeb]

Since the pre-compiled executables for linux are not provided anymore, we need to rewrite the download routines.

There is an ongoing PR to provide builds with GitHub:
ufz/ogs5#164

Beside that, it could be a good idea to provide an OGS5 conda package.

[MuellerSeb]

Fixed by removing the Jenkins dependent downloads. Only 5.7, 5.7.1 and 5.8 are possible now.