Remove postmarketos-mvcfg

I've added the postmarketos-mvcfg scripts in the past as an attempt to migrate from configs created with legacy packaging methods where we just wrote to /etc/ files with postinstall scripts. See https://wiki.postmarketos.org/wiki/Packaging/Override_configuration_files for details.

postmarketos-base depends on this.

But it seems that the scripts are just a source of confusion when found in APKBUILDs / postinstall scripts.

Also they are barely used.

So I think we should look into the few cases where they are actually used (only in devicepkg-utils and device-nokia-n900 apparently), write an edge blog post noting that users will need to take manual action from now on, when upgrading a years old install that still has such a legacy config in /etc. And then remove postmarketos-mvcfg.

TODO:

write edge blog post
remove postmarketos-mvcfg packages and usage in pmaports
update wiki pages (remove them?)

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