[UnnecessaryFunctionArguments] Warning on callback implementations

[pbrudnick]

Bug Description

The UnnecessaryFunctionArguments rule is analyzing callback implementations which don't have set @impl before.

To Reproduce

I have this module implementing Plug:

defmodule CoophubWeb.Plug.SubdomainMatcher do
  @behaviour Plug
  import Plug.Conn, only: [put_private: 3, halt: 1]
  import Phoenix.Controller, only: [redirect: 2]

  @spec init(Keyword.t()) :: Keyword.t()
  def init(_opts) do
    Application.get_env(:coophub, CoophubWeb.Endpoint)[:url][:host]
  end

REDACTED
end

And I get:

lib/coophub_web/plug/subdomain_matcher.ex:7 - In module CoophubWeb.Plug.SubdomainMatcher: Argument in position 1 of CoophubWeb.Plug.SubdomainMatcher.init/1 is ignored in all of its clauses

Expected Behavior

Ignore callback implementations without @impl (maybe functions with same name/arity used in the module) or suggest to add it. Besides, e.g, the Plug docs don't suggest that thing https://hexdocs.pm/plug/readme.html#hello-world