Remove obsoleted sectional_codeowners feature flag in test

As part of work to iteratively build section codeowners (#12137 (closed)) I did development work behind a feature flag sectional_codeowners which can be removed now that sectional codeowner processing has aligned with the legacy processing - feeding a sectional or legacy formatted file through either the legacy or sectional processing routines will return the same data (the only difference being the sectional data through the sectional parser will add add to the Entry object the section in which it was found.)