HLTRates2COOL: delete obsolete package
This code was used during Run-1 to store HLT rates in COOL (in Run-2 we used https://gitlab.cern.ch/atlas-tdaq-software/HLTRates2Cool, which is also obsolete as we no longer store HLT rates in COOL, see discussion in ATR-15588).
Relates to ATR-19150.