Remove unused require and unused model stub

- The `Project` model should have been removed in 468939297d.
- The superfluous require was added in 605c6455ac.

Closes #25215

Signed-off-by: Jeremy Daer <jeremydaer@gmail.com>

activemodel/test/cases/helper.rb

@@ -1,5 +1,4 @@
 require 'active_model'
-require 'active_support/core_ext/string/access'
 
 # Show backtraces for deprecated behavior for quicker cleanup.
 ActiveSupport::Deprecation.debug = true

activemodel/test/models/project.rb

@@ -1,3 +0,0 @@
-class Project
-  include ActiveModel::DeprecatedMassAssignmentSecurity
-end