Remove StringBuilder::toStringPreserveCapacity

This API uses StringImpl::m_substringBuffer, which we're in the process of
removing. We could re-implement this API using StringView, but none of the
callers of this API seem to actually require this optimization.

R=eseidel@chromium.org
BUG=262320

Review URL: https://codereview.chromium.org/19803008

git-svn-id: svn://svn.chromium.org/blink/trunk@154589 bbb929c8-8fbe-4397-9dbb-9b2b20218538