Remove unneeded static cache in file_save_upload()

Problem/Motivation

In #3375423: Deprecate file_managed_file_save_upload(), file_save_upload() and _file_save_upload_from_form() and replace with a service we are modernising file uploads. The static $upload_cache; seems unnecessary, and it is worth checking this before refactoring the code.

The comments say:

" // Return cached objects without processing since the file will have
// already been processed and the paths in $_FILES will be invalid."

This issue is to evaluate whether this is still needed.

Steps to reproduce

Remove the cache and check if tests still pass.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet