util/disk_cache: Do not try to delete old cache if cache is disabled
util/disk_cache: Do not try to delete old cache if cache is disabled
Prevent following warning if not running as a normal user:
Failed to create /home for shader cache (Permission denied)---disabling
disk_cache_delete_old_cache() is going to create first the cache directory using disk_cache_generate_cache_dir(). From mkdir_if_needed(), the stat() of "/home" fails with "Permission denied" under some circumstances when using Firefox.
This problem was reported in the following Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1921742
Also update documentation to match current behavior.