Delete the legacy TriggerFlags.doHLT. There are only very few
use-cases where a doHLT flag is even needed and those are already
covered by ConfigFlags.Trigger.doHLT (False by default).
This removes the last uses of TriggerFlags from runHLT_standalone
(ATR-21276).
Also delete the obsolete writeBS Modifier and its include file.
Needs to wait for !47410 (merged).