Remove unused @epoch module attribute in integration test (#6816)

In commit 36961b6
([#6764](https://redirect.github.com/phoenixframework/phoenix/pull/6764)),
the test "development workflow works as expected" was updated to
"generated app boots with mix phx.server", removing the `File.touch!`
and `File.stat!` assertions on non-existent HTML templates in an app
generated with `--no-html`.

However, the `@epoch` module attribute definition at the top of the file
was left behind, causing the compiler warning:
  "warning: module attribute @epoch was set but never used"

This removes the unused `@epoch` definition.

Author: rhcarvalho

integration_test/test/code_generation/app_with_no_options_test.exs

@@ -1,8 +1,6 @@
 defmodule Phoenix.Integration.CodeGeneration.AppWithNoOptionsTest do
   use Phoenix.Integration.CodeGeneratorCase, async: true
 
-  @epoch {{1970, 1, 1}, {0, 0, 0}}
-
   test "newly generated app has no warnings or errors" do
     with_installer_tmp("app_with_no_options", fn tmp_dir ->
       {app_root_path, _} =