[12.x] Check if file exists before trying to delete it

[Jellyfrog]

When having the error_reporting set to showing warnings, this avoids the warning "No such file or directory" if the file doesn't exist.

For example:

$ php artisan config:clear
unlink(/home/jellyfrog/code/librenms/bootstrap/cache/config.php): No such file or directory in /home/jellyfrog/code/librenms/vendor/laravel/framework/src/Illuminate/Filesystem/Filesystem.php on line 308

   INFO  Configuration cache cleared successfully.

NOTE: This is not a 100% correct fix, in Linux you should check if you have write+execute permissions on the parent-folder, but since there is a try/catch-block and the @-suppression already, I believe it's a good enough middle ground. Let me know if you want the more correct fix instead.

[rodrigopedra]

This should be reverted, as

php artisan storage:link --force

no longer works.

The added is_file() check by this PR returns false for a symlink, therefore any existing links are not deleted here:

framework/src/Illuminate/Foundation/Console/StorageLinkCommand.php

Lines 42 to 44 in c86346f

	if (is_link($link)) {
	$this->laravel->make('files')->delete($link);
	}

ping @taylorotwell @crynobone

I can't work on a PR right now.