Remove actix-rt depenency

[Nutomic]

If you create an application with tokio runtime which depends on this library, it crashes at startup with the following error:

thread 'main' panicked at 'System is not running', /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/actix-rt-2.8.0/src/system.rs:120:211
stack backtrace:
0: std::panicking::begin_panic
at /rustc/84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc/library/std/src/panicking.rs:611:12
1: actix_rt::system::System::current::{{closure}}
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/actix-rt-2.8.0/src/system.rs:120:21
2: std::thread::local::LocalKey<T>::try_with
at /rustc/84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc/library/std/src/thread/local.rs:446:16
3: std::thread::local::LocalKey<T>::with
at /rustc/84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc/library/std/src/thread/local.rs:422:9
4: actix_rt::system::System::current
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/actix-rt-2.8.0/src/system.rs:118:9
5: actix_rt::arbiter::Arbiter::with_tokio_rt
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/actix-rt-2.8.0/src/arbiter.rs:115:19
6: actix_rt::arbiter::Arbiter::new
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/actix-rt-2.8.0/src/arbiter.rs:101:9
7: background_jobs_actix::Manager::new
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/background-jobs-actix-0.13.1/src/lib.rs:178:31
8: background_jobs_actix::WorkerConfig<State,background_jobs_actix::Managed>::start
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/background-jobs-actix-0.13.1/src/lib.rs:357:9
9: activitypub_federation::activity_queue::create_activity_queue
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/activitypub_federation-0.4.0/src/activity_queue.rs:229:5
10: activitypub_federation::config::FederationConfigBuilder<T>::build
at /home/felix/.cargo/registry/src/github.com-1ecc6299db9ec823/activitypub_federation-0.4.0/src/config.rs:179:21
...

This is caused by the background-jobs crate. It would be good to get rid of this requirement, so that any async runtime can be used.