Twenty Seventeen: Make gallery link styles consistent in all widget areas

Removes the box shadow on gallery links in footer widget areas so they're consistent with the sidebar widget area.

Props transl8or, sami.keijonen, davidakennedy.

Fixes #42302.

Built from https://develop.svn.wordpress.org/trunk@42004


git-svn-id: https://core.svn.wordpress.org/trunk@41838 1a063a9b-81f0-0310-95a4-ce76da25c4cd