Optimize deps: Drop `error_chain` and move `assert_approximate_eq` to dev-deps

[w-flo]

• [X] I agree to follow the project's code of conduct.

• [ ] I added an entry to CHANGELOG.md if knowledge of this change could be valuable to users.

  Well, first of all, thanks again for the gpx crate! I've noticed the latest version parses gpx files that use some extensions without throwing an error, and I can access the time without going through a parse/format cycle, so it's even better now compared to last time I participated in this repo.

I'm not sure how valuable a changelog entry for this would be for users, and English is not my native language. So I haven't added one. Let me know if I should, or feel free to add it yourself if you want :-)

I've noticed these deps do not appear to be needed. The error_chain dependency seems especially "wasteful" because it brings in a bunch of transitive dependencies. Commit messages:

---

Move `assert_approx_eq` crate to dev-dependencies

The crate is used in two test cases and appears not to be needed
otherwise.

---

Remove error_chain remnants

The code no longer depends on error_chain since #58, but the dependency
was still listed in Cargo.toml. There still was one commented-out "use"
directive.

Drops the number of build steps from 69 to 48, and helps with
compilation time a bit.

[michaelkirk]

Thanks!

bors r+

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