TrigKernel: Further clean-up and make it a header-only library
Further clean-up of the TrigKernel package following up from !29024 (merged):
- Remove HltExtraStatusCode - although it has been possibly recorded in some Run-2 data, it has never been used and it seems no decoding of these codes has ever been implemented in Athena
- Remove HltOnlHelper which was only serving the purpose of enum->string mapping
- Implement much cleaner way of enum->string mapping for the remaining enum PSCErrorCode
- Adapt clients to the above change
- Thanks to the above, it was now possible to make TrigKernel a header-only library