Revert "add private non-copying SkReadBuffer::readString()"

This reverts commit 232783f0c5c9e3244c977554e368c5a07768d397.

Reason for revert: several fuzzers find nullptr deref.  Will investigate and reland.

Original change's description:
> add private non-copying SkReadBuffer::readString()
> 
> ... and use it when looking up deserialized type factories.
> 
> Change-Id: I23d6dd549d4258908a961971a8de380776c91e1c
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/171220
> Commit-Queue: Mike Klein <mtklein@google.com>
> Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>

TBR=mtklein@google.com,khushalsagar@chromium.org

Change-Id: I552f8de413dd52e1480c8ce1e7e84a072a54cb93
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/200940
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>