Flushing cache should not require `wp-cache-config.php` being writable

I keep `wp-cache-config.php` read-only for PHP's user, because I want all changes to configuration files to be properly tracked in version control, which is a best practice.

Doing that prevents admins from clearing the cache, though. It doesn't seem like there's any reason why Super Cache would need to write to `wp-cache-config.php` in order to clear the cache.

If the config file isn't writable, then `wp_cache_manager_error_checks()` defines `SUBMITDISABLED` so that the `Delete Cache` button is disabled.

But calling the prune function manually works even with the file unwritable:

```
> wp shell --url=https://foo.example.org
wp> require_once( 'wp-content/plugins/wp-super-cache/wp-cache-phase1.php' )
wp> prune_super_cache( 'wp-content/cache/supercache/foo.example.org', true )
=> int(50)
```

Read wp_cache_manager_error_checks() first, then compare its SUBMITDISABLED path with the manually demonstrated prune_super_cache() call in wp-cache-phase1.php. Done means an unwritable wp-cache-config.php no longer disables Delete Cache while cache pruning still works.

Written by the indexing model from the issue text.