api: remove legacy password_hash login method.

Remove the ability to authenticate to the API with the `login` and
`password_hash` url parameters. This is a legacy authentication method
from Danbooru 1. How to actually generate the password_hash for this
method hasn't been fully documented for many years now. It required
taking the SHA1 hash of your password combined with an undocumented salt
value (i.e., password_hash = sha1("choujin-steiner--#{password}")).

This authentication method was also slow because it required checking
the password on every API call. Checking passwords is deliberately slow
because passwords are hashed with BCrypt. BCrypt takes about ~200ms per
request, so using this method effectively limited you to ~5 requests per
second in a single thread.

app/models/user.rb

@@ -181,17 +181,6 @@ class User < ApplicationRecord
     def hash_password(password)
       Digest::SHA1.hexdigest("choujin-steiner--#{password}--")
     end
-
-    module ClassMethods
-      def authenticate_hash(name, hash)
-        user = find_by_name(name)
-        if user && user.bcrypt_password == hash
-          user
-        else
-          nil
-        end
-      end
-    end
   end
 
   module LevelMethods