This replaces a site ID comparison when displaying action links in `WP_MS_Sites_List_Table::handle_row_actions()` with a dedicated function call, `is_main_site()`, for clarity.
Follow-up to [13918], [25125], [26120], [32644], [38814].
Props ecorica, spacedmonkey, SergeyBiryukov.
Fixes#58150.
Built from https://develop.svn.wordpress.org/trunk@55666
git-svn-id: http://core.svn.wordpress.org/trunk@55178 1a063a9b-81f0-0310-95a4-ce76da25c4cd