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, 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. 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
issue