Coding Standards: Use strict comparison in `wp-admin/user/admin.php`.

Apply the case-insensitive network-site comparisons added in [27725] for `wp-admin/network/admin.php` to `wp-admin/user/admin.php` too, for consistency.

Props rimadoshi.
Fixes #49300.
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git-svn-id: https://core.svn.wordpress.org/trunk@46915 1a063a9b-81f0-0310-95a4-ce76da25c4cd