Fix build with "ring" instead of "aws_lc_rs" feature

[procr1337]

Since there are feature flags for ring and aws-lc in libunftp, it seems unintended to pull aws-lc-sys unconditionally via the tokio-rustls default features.

Furthermore, the build failed with "aws_lc_rs" disabled, since tls.rs unconditionally imported rustls::crypto::aws_lc_rs. With this PR merged, there will be a meaningful error message if neither "ring" nor "aws_lc_rs" are enabled, and enabling "ring" without "aws_lc_rs" is supported again.

Last commit just removes some dead code which caused a warning.