Move mcumgr code back to core

At time mcumgr was moved to separate repo [1] and were enhanced with adaptation layer to help useage by other embedded projects.
After some time it turns out that projects prefer to have own fork instead, so this is no longer needed.

- move code back to core
- remove non-mynewt code
- remove adaptation layer
- archive [1]

[1] https://github.com/apache/mynewt-mcumgr

Begin with the mcumgr code in the separate repository linked as [1] and compare it with the mynewt-core tree. Identify the non-Mynewt code and adaptation layer that the issue says to remove before planning the move. Done means the relevant code is back in core, those parts are removed, and the separate repository is archived.

Written by the indexing model from the issue text.