Remove V8PerIsolateData::m_lazyEventListenerToStringTemplate

Because it's never used. Note that V8PerIsolateData::lazyEventListenerToStringTemplate() always returns an empty handle.

I don't think it's worth caching the template, since the method is used by not-performance-sensitive code, so this CL simply removes it.

Review URL: https://codereview.chromium.org/156873007

git-svn-id: svn://svn.chromium.org/blink/trunk@166991 bbb929c8-8fbe-4397-9dbb-9b2b20218538