Remove SkipDotsRecursiveDirectoryIterator in favor of builtin FilesystemIterator::SKIP_DOTS flag

In #935036: File exists in FileTransferFTPExtension->createDirectoryJailed(), we subclassed RecursiveDirectoryIterator to provide an iterator which skips '.' and '..' files. This was necessary since there was no built-in mechanism in PHP 5.2 to accomplish this. In PHP 5.3 RecursiveDirectoryIterator's parent was changed to FilesystemIterator which provides a convenient SKIP_DOTS flag.

This means we can remove the SkipDotsRecursiveDirectoryIterator compatibility layer and replace all calls like
new SkipDotsRecursiveDirectoryIterator($path) with new RecursiveDirectoryIterator($path, FilesystemIterator::SKIP_DOTS).

Since this requires PHP 5.3, this is for Drupal 8 only. I'm filing under update.module, but the relevant code all lies under includes/filetransfer/....