Remove multithreading support

[oli-obk]

cool side effect: our dependency graph is now

secs v0.1.0 (C:\Users\oli\secs)
└── elsa v1.11.0
    └── stable_deref_trait v1.2.0

instead of

secs v0.1.0 (C:\Users\oli\secs)
├── elsa v1.11.0
│   └── stable_deref_trait v1.2.0
└── parking_lot v0.12.3
    ├── lock_api v0.4.12
    │   └── scopeguard v1.2.0
    │   [build-dependencies]
    │   └── autocfg v1.4.0
    └── parking_lot_core v0.9.10
        ├── cfg-if v1.0.0
        ├── smallvec v1.14.0
        └── windows-targets v0.52.6
            └── windows_x86_64_msvc v0.52.6

[wick3dr0se]

Sick! It's a hell of a lot more simple that's for sure lol