db: drop unused columns from users table.

Note that the password_hash column was replaced by bcrypt_password_hash
in 2013, but the old password_hash column was never dropped.

db/migrate/20191116224228_drop_unused_columns_from_users.rb

@@ -0,0 +1,10 @@
+class DropUnusedColumnsFromUsers < ActiveRecord::Migration[6.0]
+  def change
+    execute "set statement_timeout = 0"
+
+    remove_column :users, :base_upload_limit, :integer, null: false, default: 10
+    remove_column :users, :recent_tags, :string
+    remove_column :users, :email_verification_key, :string
+    remove_column :users, :password_hash, :string, null: false
+  end
+end