clean up some unused parts of SkArenaAlloc

I'm looking at making SkArenaAlloc independent of Skia.
Here's a bit of low-hanging fruit.

Nothing is using makeSkSp().  Good... it seems real dangerous.

Change-Id: Ib7154e7948a3c6d828376ef37935636b3b4695ff
Reviewed-on: https://skia-review.googlesource.com/132824
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
3 files changed