1) It's unused. 2) Since Zones can be used on a background thread, the Zone
cannot just access this counter from the Isolate, so keeping this counter up to
date would require extra work. Better to just remove it.

BUG=431860
LOG=n
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/716793002

Cr-Commit-Position: refs/heads/master@{#25263}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25263 ce2b1a6d-e550-0410-aec6-3dcde31c8c00