Description of problem:
check-katello-agent-enabled is not valid for 6.15.z scenario
this pre-upgrade check is only valid for 6.15 scenario

Version-Release number of selected component (if applicable):
6.15.0

How reproducible:
deterministic

Steps to Reproduce:
1. Have Sat 6.15 (=katello-agent has been already removed prior 6.15 upgrade)
2. Run upgrade (or pre-upgrade check) across 6.15.z zstream

1. satellite-maintain upgrade check --target-version 6.15.z -w repositories-validate -y
   ...
   --------------------------------------------------------------------------------
   Check whether the katello-agent feature is enabled before upgrading: [OK]
   --------------------------------------------------------------------------------

Actual results:
check for katello-agent feature is performed

Expected results:
check for katello-agent feature is removed for 6.15.z scenario

Additional info: