library/MNCC_EncDec: rip out MNCC version 6 support

The latest release of osmo-sip-connector (1.5.0, 23 Feb 2021) does
support MNCCv7, so there is no need to maintain MNCCv6 support.

Change-Id: Ie45158e805a62e86b9496b46f323b83a74630460
Related: I5448ff931ec33f24f4837a51376f1703fe97683b
Related: OS#5282
2 files changed