remove SkWriteBuffer::reserve()

We're not using it for anything that we can't do using the clearer writeFoo()/readFoo() methods.

The deletions in SkPictureFlat.{h,cpp} and BitmapHeapTest.cpp clean up dead code.
This code has not been used for a long time.  I happened to notice it because it
was another caller of reserve() (in a terrifyingly hacking/interesting way that
I bet we'll find I authored...)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1930103003

Deleting APIs unused publicly.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/1930103003
5 files changed