Revert "core: enable TasksMax= for all services by default, and set it to 512"

Revert "core: enable TasksMax= for all services by default, and set it to 512"

This reverts commit 9ded9cd14cc03c67291b10a5c42ce5094ba0912f.

Introducing a default limit on number of threads broke a lot of software which
regularly needs more, such as MySQL and RabbitMQ, or services that spawn off an
indefinite number of subtasks that are not in a scope, like LXC or cron.

15% is way too much for most "simple" services, and it's too little for others
such as the ones mentioned above. There is also no particular rationale about
any particular global limit, so even if we'd bump it higher we'd just make the
limit even less useful while still breaking software.

It is both much safer and also much more effective in terms of guarding against
berserk programs/bugs/unintended fork bombs etc. to set limits in units
individually. Once someone looks at one, this is then a great time to also flip
on the other resource and privilege limitations that systemd offers.

Bug: https://github.com/systemd/systemd/issues/3211
Bug-Debian: https://bugs.debian.org/823530
Bug-Ubuntu: https://launchpad.net/bugs/1578080

Gbp-Pq: Topic debian
Gbp-Pq: Name Revert-core-enable-TasksMax-for-all-services-by-default-a.patch