remove sync-spin and sync-parking_lot

[arnauorriols]

These Mutex implementations are for sync code, and Streams is async. We should implement these for tokio::sync::Mutex, futures::sync::Mutex, and the like. However, what are the use case of these? Why can't people just use whatever they want, when implementing a custom Transport?

[kwek20]

https://docs.rs/mutex-trait/latest/mutex_trait/
-> Doesnt do well with the trait in trait, only for fields&params (We could make our own trait)

https://docs.rs/maybe-async/0.2.6/maybe_async/index.html
-> Didnt like the lifetimes we need to keep the async_trait

Could change the trait based on #[cfg(not(feature = "sync"))], but id need to make it for tangle and bucket as well (messy)

Could just remove it.. who doesnt use async nowadays? its even no-std 🤷‍♂

Or just implement a switch for all possible Mutexi... (muti? Mutexes?)

[DyrellC]

I'd personally be more in favor of removal, every other aspect of the implementation is async, i'm not sure I see much of a benefit in retaining sync based Mutex's in a fully async implementation.