wasi-common: remove wasmtime dependency from sync crate feature

[Robbepop]

Closes #8894.

Motivation: This allows usage of wasi-common in non-Wasmtime Wasm runtimes.

This was probably an oversight in #7881.

The only optional usage of the wasmtime crate that I could find within the wasi-common/sync module is the following:

#[cfg(feature = "wasmtime")]
super::define_wasi!(block_on);

Link: https://github.com/bytecodealliance/wasmtime/blob/main/crates/wasi-common/src/sync/mod.rs

Which defines the add_to_linker methods for Wasmtime which is entirely optional.
Non Wasmtime users of wasi-common might want to define their own add_to_linker functions.

Locally I did not see any compile errors with the removal of this dependency of this PR.

[alexcrichton]

Thanks!