Drop dmail filters.

Few people used dmail filters (~900 users in 5 years) and even fewer
used them correctly. Most people used them to try to block dmail spam,
but usually they either blocked too much (by adding common words that
are present in nearly all dmails, causing all mails to them to be
filtered) or too little (blocking specific email addresses or urls,
which usually are never seen again after the spammer is banned).
Nowadays the spam detection system does a better job of filtering spam.

app/models/user.rb

@@ -107,7 +107,6 @@ class User < ApplicationRecord
   has_one :recent_ban, -> {order("bans.id desc")}, :class_name => "Ban"
 
   has_one :api_key
-  has_one :dmail_filter
   has_one :super_voter
   has_one :token_bucket
   has_many :notes, foreign_key: :creator_id
@@ -122,7 +121,6 @@ class User < ApplicationRecord
   has_many :tag_aliases, foreign_key: :creator_id
   has_many :tag_implications, foreign_key: :creator_id
   belongs_to :inviter, class_name: "User", optional: true
-  accepts_nested_attributes_for :dmail_filter
 
   enum theme: { light: 0, dark: 100 }, _suffix: true