Remove FileSizeLimitCheck CE stub and consolidate into EE

Summary

lib/gitlab/checks/file_size_limit_check.rb exists as a hollow stub with all real logic already living in ee/lib/ee/gitlab/checks/file_size_limit_check.rb. The CE file should be removed and the EE file updated to be self-contained.

Background

In !187085 (merged), the validate! logic was moved from CE to EE, leaving the CE class as an empty stub:

def validate! nil end

The EE module fully overrides validate! without calling super, making the CE stub dead code. The flaky CE spec was removed in !225032 (merged). This issue tracks the remaining cleanup of removing the CE stub itself.

Proposal

• Remove lib/gitlab/checks/file_size_limit_check.rb
• Update ee/lib/ee/gitlab/checks/file_size_limit_check.rb to be a standalone class instead of a module that prepends onto a CE class
• Update any references/autoloading as needed