Valid and safe block style variations were being removed by
`WP_Theme_JSON_Gutenberg::remove_insecure_properties` when securing the
theme.json. When this was a problem varied depending upon site
configuration, but out-of-the-box it was a problem for administrators on
multi-site installs.

This change adds explicit processing of variations in
`remove_insecure_properties` so that they won't get removed.

This test checks that when removing insecure properties an
unknown/unsupported property is removed from the variation.