www_stream: use DESTROY to cleanup temporary gits

www_stream: use DESTROY to cleanup temporary gits

Relying on a timer to handle cleanup in f9ac22a4b485 was
sub-optimal since the delay could prove expensive under heavy
traffic.  So rely on ->DESTROY instead since we we no longer
hold reference cycles by the time the show_blob callback
executes.

Fixes: f9ac22a4b485 ("git_async_cat: automatically cleanup temporary gits")