Remove AMP-illegal overflow values (auto, scroll)

[westonruter]

The new validity reporting in ampproject/amp-wp#971 brought to light that the overflow property in this theme had illegal values according to the spec:

overflow (and overflow-y, overflow-x) may not be styled as “auto” or “scroll”. No user defined element in an AMP document may have a scrollbar.

Since they are getting stripped by the sanitizer anyway, it seems they should be removed from the CSS. Alternatives should be explored if needed.

[westonruter]

Without this change, the AMP validation utils will continue to complain with:

[westonruter]

This will no longer be required as of ampproject/amp-wp#1048