Allow use of public API from C++

Signed-off-by: Andrey Mazo <mazo@telum.ru>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Remove unused functions

After 95ee490bbdb97ab2b4f1dfa63a0a26e0dd1c2f17 they are not in public
API and are not used throughout the lxc codebase.
This has a bonus of removing workaround for bionic.

Signed-off-by: Andrey Mazo <mazo@telum.ru>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>