Clean up `use_introspect_parser` feature flag related logic

Why are we doing this work

This issue is to clean up the logic related to the use_introspect_parser feature flag as it's been enabled by default in %15.6.

Relevant links

• FF rollout issue: [Feature flag] Enable `use_introspect_parser` (#381789 - closed)
• FF implemented by: Save `finding_data` for security findings (!103013 - merged)

Implementation plan

• Run /chatops run feature delete use_introspect_parser --dev --staging --staging-ref --production to delete the FF
• backend Remove the use_introspect_parser FF-related logic from ee/lib/ee/gitlab/ci/parsers/security/common.rb to always use introspect_parser. Basically removing the report_data from this EE module and adding the introspect_parser to the CE module should be enough.
• backend Remove the FF definition from the codebase.