This PR combines the wasi-tls crates back into a single crate with feature flags.

The current setup was never the desired state, but was a workaround to make CI work back when nativetls support was first added:

Hm ok so this is actually going to be kind of tricky. We test crates in CI with --all-features but we also test cross-compilations to various architectures, and native-tls on Linux wants OpenSSL which wants a system copy by default which isn't present for cross-compiled variants.
(..) ideally we wouldn't test native-tls by default and we'd instead have a dedicated job for just testin native-tls, but with --all-features that's not easy :(

In this PR I work around the cross-compilation issue by simply not testing wasi-tls in run-tests.py at all. Testing wasi-tls is now handled by a dedicated CI job.

This change effectively abandons the wasmtime-wasi-tls-nativetls & wasmtime-wasi-tls-openssl crates on crates.io. Don't know if this need to be communicated somehow?