Deprecate system_check_directory()

Problem/Motivation

system_check_directory() is only used in one place in core, in locale_form_system_file_system_settings_alter(), but its loaded on every request.

http://codcontrib.hank.vps-private.net/search?text=system_check_director...

Proposed resolution

Deprecated it without replacement because there's only 8 usages found in contrib

Remaining tasks

- patch/review
- update CR
- commit

User interface changes

no

API changes

The system_check_directory() is deprecated

Data model changes

no

Release notes snippet

no