Login: Revert [34213] and [35897]. It has become apparent that there is a need...

Login: Revert [34213] and [35897]. It has become apparent that there is a need for a separate function (and corresponding filter) which allows for the login form action URL to differ from the URL used to access the login form, so that plugins or implementations which change the login URL do not need to worry about handling the form submission at the same URL.

For now, we'll revert to the pre-4.4 behaviour of hard-coding the login form action URL as `wp-login.php` and look at implementing a separate function and corresponding filter in 4.5.

Props KrissieV, salcode, JPry
Fixes #34925
See #35103

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