TriggerJobOpts: cleanup TriggerConfigGetter and flags

- Replace `TriggerFlags.dataTakingConditions` by
  `ConfigFlags.Trigger.availableRecoMetadata`: this flag is used to indicate
  the trigger systems that are enabled during reconstruction. It is set
  based on the L1/HLT keys in COOL or the metadata in the input file.
- Remove obsolete metadataItems setting (the key names are not relevant
  anymore, and we are setting this elsewhere already).
- Deprecate `TriggerFlags.configForStartup`: this flag was used to
  indicate whether trigger keys can be found in COOL. But this is
  essentially the same check as `availableRecoMetadata` performs now.

I did not re-implement the setting of the `DQMonFlags` based on the
available systems. If this is really needed still for the legacy
monitoring it should be done in the relevant DQ job options.