Remove obsolete model

It was replaced by Dashboard::AdministratorTask in commit e5f9cf67.

app/models/administrator_task.rb

@@ -1,11 +0,0 @@
-class AdministratorTask < ApplicationRecord
-  belongs_to :source, polymorphic: true
-  belongs_to :user
-
-  validates :source, presence: true
-
-  default_scope { order(created_at: :asc) }
-
-  scope :pending, -> { where(executed_at: nil) }
-  scope :done, -> { where.not(executed_at: nil) }
-end