Remove the configuration files for plugin loading

[Vojtech Trefny]

We should investigate possibility of removing the config files in data/conf.d that control what plugin type and version will be loaded. I doubt that people are actually using that -- both udisks and blivet are happy with whatever is default except for LVM where we explicitly require the cli plugin -- and it has problems like #926 that we could easily avoid. Thoughts @vpodzime @tbzatek

[Tomáš Bžatek]

I vote for making libblockdev as stateless (in terms of config files) as possible. Defaults can be compiled-in and only overrides can be taken in account from /etc.

[Vratislav Podzimek]

Yup, there should be defaults. But I think we should also support the config files to override the defaults.