Drop commit_flags_uid_key from commit_flags
    
    We enforce the commit flag to be unique per PR or per commit.
    We removed it from the PR flags table in dc4c195ceb1b399676a0dca5da9e72914bea1241
    but we didn't remove it from the commit flags table... until now.
    
    Fixes https://pagure.io/pagure/issue/4446
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>