Move master slots to LCG104
Moved master slots to LCG104, including dependent ones like run3-cleanup. Changed list of platforms accordingly. At this stage lhcb-master has a rather restricted list of platforms as it's also used for ci-tests where we have limited power and the full set is in lhcb-head.
The goal in a second step is to have all platforms in master and have master-mr be a subset of master.
Several cleanups on the way :
- dropped several unused slots (gaudi-merge, coverity, head-2, dd4hep, prerelease, online)
- simplified the list of plaforms used