Remove system.file.path config from system.schema.yml

Problem/Motivation

In #3400368: Deprecate path.temporary in system.file configuration schema (Drupal 8.8) system.file.path.temporary was deprecated. This was the only key used in system.file.path, but that key in the schema was not removed.

Lets use this issue to deprecate the config key and add an upgrade path.

Steps to reproduce

Proposed resolution

1. Remove system.file.path from system.schema.yml.
2. Create an upgrade path to remove the key.
3. Create an upgrade path test.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet