Remove old Astarte client

As the last step of resolving #33, replace the old client package
with a new one, more idiomatic and simple.

Signed-off-by: Arnaldo Cesco <arnaldo.cesco@secomind.com>

Author: Annopaolo

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Breaking changes
+- Remove the 0.90.1 Astarte client API and introduce a clean, idiomatic API.
+  See [#33](https://github.com/astarte-platform/astarte-go/issues/33).
 ### Changed
 - Replace device `metadata` with `attributes`.