remove _is_wcs_correction_small test

[mcara]

This PR removes unit test for a private stcal method _is_wcs_correction_small and addresses spacetelescope/stcal#529 (comment) by @braingram

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[braingram]

Thanks!

[emolter]

LGTM, it does indeed seem this test should have been removed when tweakreg went to stcal