Do not silence unused variable warnings in assert_receive

Closes #4789. It's not needed anymore after e190474 commit.

Author: lexmag

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -379,7 +379,6 @@ defmodule ExUnit.Assertions do
                           <> ExUnit.Assertions.__mailbox__(self()))
         end
 
-      _ = unquote(vars) # Silence warnings
       received
     end
   end