Clean up dependency tree

[jamilbk]

• Why does cargo build -p connlib-client-apple build firezone-relay?
• Unused crate dependencies (see build(rust): Trigger warnings for unused crate dependencies #4402)
• Unused features from dependencies

It might be more tedious but maybe it's a good idea to have more target sections for each platform in our Cargo.tomls so that we aren't inadvertently linking in libraries that aren't used. It can be tough to remove them later, even relying on the linker to do so.

During our hunting session for #4377, @ReactorScram and I found lots of cross-pollination of dependencies across platforms.