users: replace scopes with associations.

app/models/dmail.rb

@@ -138,14 +138,6 @@ class Dmail < ApplicationRecord
       where("owner_id = ?", CurrentUser.id)
     end
 
-    def to_name_matches(name)
-      where("to_id = (select _.id from users _ where lower(_.name) = ?)", name.mb_chars.downcase)
-    end
-
-    def from_name_matches(name)
-      where("from_id = (select _.id from users _ where lower(_.name) = ?)", name.mb_chars.downcase)
-    end
-
     def search(params)
       q = super