mynewt: remove version.yml and references

With the merge of https://github.com/apache/mynewt-newt/pull/365

 `newt`
does not use `version.yml` files anymore, so remove the file and
references to updating it in the release process.

Testing a MCUBoot release now depends on `newt` `master` or `> 1.7.0`
(to be released).

Signed-off-by: Fabio Utzig <utzig@apache.org>