Remove `kernel_at_exit_hacks.rb`.

Library/Homebrew/cask/spec/support/kernel_at_exit_hacks.rb

-module Kernel
-  alias real_at_exit at_exit
-
-  def at_exit(&block)
-    real_at_exit(&block) unless ENV["DISABLE_AT_EXIT"]
-  end
-
-  def with_disabled_at_exit
-    ENV["DISABLE_AT_EXIT"] = "1"
-    yield
-    ENV.delete("DISABLE_AT_EXIT")
-  end
-end