Deprecate file_default_scheme() and just use config

Problem/Motivation

We want to deprecated procedural functions in file.inc and replace with methods on FileSystem or StreamWrapperManager.

file_default_scheme() just wraps \Drupal::config('system.file')->get('default_scheme'), and porting it over just creates another API we need to maintain.

Also, we are calling file_default_scheme() in a load of tests, where we really only need 'public://'

Proposed resolution

1. Replace usages of file_default_scheme() with \Drupal::config('system.file')->get('default_scheme')
2. Replace usages of file_default_scheme() in tests with 'public://'

Remaining tasks

None.

User interface changes

None.

API changes

file_default_scheme() is deprecated.

Data model changes

None.

Release notes snippet