Remove reqwest default tls feature

[flosse]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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[lnicola]

r? @urschrei

[urschrei]

This will need a rebase when #57 merges, right?

[michaelkirk]

The chrono work was just merged in #60 so this will at least need a rebase.

But also... what is this PR doing? Is it disabling TLS? Why would we want that?

[michaelkirk]

Oh, I think I understand.

We want people to be able to choose their tls backend, which are already exposed by geocoding features:

[features]
default = ["reqwest/default"] # this builds default-tls
rustls-tls = ["reqwest/rustls-tls"] # this builds rust-tls

But we were erroneously (redundantly) declaring that reqwest always builds default-tls, so there was no way to use only rust-tls without this PR. Is that right?