Revert "Parallelize template rendering"

This reverts commit 1ad71592.

Since it had no limit on the number on concurrent processes it OOM killed instances while rendering hellthreads. When I tried introducing a concurrency limit with Task.async_stream/manual folds it lead to about 3 times worse performance on threads larger than 1000 activities (we are talking 30s vs 1.2 minutes), I think this is not worth the about 1.5 times performance increase on smaller threads when using it.