shutdown: fix a crash when shutting down a worker process

Remove the deferred cleanup, as in ngx_pagespeed the fetcher
is owned by the RewriteDriverFactory.

Fixes the backtrace pointing to
net_instaweb::RewriteDriverFactory::Deleter
at https://github.com/pagespeed/ngx_pagespeed/issues/367
Merge pull request #439 from pagespeed/oschaaf-shutdown-fix

shutdown: fix segfault when shutting down a worker process