ci: add cargo-machete in the workflow

[junhochoi]

Run cargo-machete for unused dependency check, only on a default ssl target. Also removed unused dependencies recommended by an initial run of cargo-machete.

[junhochoi]

Nice! Why only run on matrix.tls-feature == ''

Because it seems redundant on other tls features. Also installing cargo-machete takes some time. Instead we can try cargo-quickinstall to save an installation time, but it will produce the same result regardless of tls-feature anyway.

[ghedo]

Merged, thanks @junhochoi! (and sorry for the delay)