Do not save the last visited tab on the multisite Network Themes page and Site...

Do not save the last visited tab on the multisite Network Themes page and Site Themes tab. These are poor UX, see #18810 for plugins. Entering these screens will always default to 'all' themes view. see #20103.

git-svn-id: https://core.svn.wordpress.org/trunk@20022 1a063a9b-81f0-0310-95a4-ce76da25c4cd