remove SkMemoryWStream -- unused externally

BUG=skia:

Change-Id: Idbc9af4e703238871f56f623cd616fb7a5e686d3
Reviewed-on: https://skia-review.googlesource.com/6727
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
15 files changed