Draft: Disable lhcb-dd4hep slot

As discussed at PAC on 21st July 2023, this slot is no longer needed as we build and test for DD4HEP in the regular nightly slots

requested review from @clemenci and @gcorti

@gcorti @clemenci as discussed at PAC, this MRs disables (but does not remove) the lhcb-dd4hep slot. If you prefer we can remove also the code to configure the slot, let me know.

@cattanem, @clemenci I am so sorry but somehow I did not realise we were talking about the lhcb-dd4hep slot!! This is still needed as is the one we are using to check the restructuring of Detector! So it will have to wait for a bit still.

My apologies for somehow missing this at PAC this morning.

marked this merge request as draft