Remove added query parameters after login redirect

[jkbcz]

Makes following changes to improve robustness of the login redirect

1. Use redirects without schema - on some PHP environments (kinsta) $_SERVER['REQUEST_SCHEMA'] is not available, so to make sure redirects work in those environments we omit schema from redirect links
2. Going directly to /wp-login.php redirects to home page after login, not wp-admin - This behavior used to depend on whether permalink structure was set to plain or pretty; this difference doesn't make sense and we should consolidate on always redirecting to home page, if someone wants to directly access the admin panel they should go to /wp-admin
3. Only use host for validating redirects, this had to be done, so the redirects without schema work.

[rvanoord]

I'm not sure "schemaless" urls are supported by all older browsers. Suggest defaulting to https:// if the scheme can't be identified