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
