Stop calling `file-attributes` for remote buffers

Hi, just a simple fix for SML making Emacs really slow when editing remote buffers.

With certain TRAMP method (in my case SFTP) and slow connections calling `file-attributes` is expensive, especially when doing it every redraw. For me it makes Emacs almost hang and unusable.

Here is the CPU profile: https://gist.github.com/tungd/a73f5ea4eca49bc43592

Thanks.