Remove sk_ref_sp's misleading preprocessor define and comment

This function will always be useful. Don't make it look/feel like a
temporary hack.

BUG=skia:

Change-Id: I6506d7d51dc3b25a7dbcea4ac273f51cf05f3a89
Reviewed-on: https://skia-review.googlesource.com/6330
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed