Revert "Add trace event for MemoryCache::prune()."

This reverts commit 432f2a91e7beedf020c36dd5dd67392f1800dd47.

It triggered some bug and caused bug 271440.
Revert this first while we'll investigate. Will reland after the
underlying bug is resolved.

BUG=271440
TBR=senorblanco
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/23031003

git-svn-id: svn://svn.chromium.org/blink/trunk@156035 bbb929c8-8fbe-4397-9dbb-9b2b20218538