Remove ext/mbedtls-asn1

[d3zd3z]

This directory was intended to be a snapshot of the ASN.1 code from Mbed TLS. With newer versions, this gets fairly large, as there are lots of include files that get brought in. Instead of this, just rely on the mbed TLS library being provided externally when ASN.1 parsing is needed.

[de-nordic]

Do we even need the ASN.1 ? As far as I understand it is used because we compile the key in such form, but why not just compile the raw key, without the encoding? Saves us code, saves us data, saves us run-time decoding.