Remove MergeRequests::Mergeability::CheckDeniedPoliciesService mergability check

Why are we doing this work

In scope of this issue we would like to remove MergeRequests::Mergeability::CheckDeniedPoliciesService as it is no longer valid and needed, because it was a part of License-Check feature that was deprecated and now all License Scanning Approvals are handled by Scan Result Policies.

Implementation plan

• Remove MergeRequests::Mergeability::CheckDeniedPoliciesService class and update the specs that depend on it

Verification steps