Problem: 0-sized protocols still emit warning

Solution:
 - Remove the warning that breaks generality.

lib/elixir/lib/protocol.ex

@@ -768,14 +768,6 @@ defmodule Protocol do
     callbacks = :maps.keys(callback_metas)
     functions = Module.get_attribute(env.module, :__functions__)
 
-    if functions == [] do
-      warn(
-        "protocols must define at least one function, but none was defined",
-        env,
-        nil
-      )
-    end
-
     # TODO: Convert the following warnings into errors in future Elixir versions
     :lists.map(
       fn {name, arity} = fa ->

lib/elixir/test/elixir/protocol_test.exs

@@ -288,13 +288,6 @@ defmodule ProtocolTest do
   describe "warnings" do
     import ExUnit.CaptureIO
 
-    test "with no definitions" do
-      assert capture_io(:stderr, fn ->
-               defprotocol SampleWithNoDefinitions do
-               end
-             end) =~ "protocols must define at least one function, but none was defined"
-    end
-
     test "when @callbacks and friends are defined inside a protocol" do
       message =
         capture_io(:stderr, fn ->