bootctl: remove all our non-volatile variables on uninstall

bootctl: remove all our non-volatile variables on uninstall

Grepping around showed a few extra entries that are not listed in the
remove_loader_variables() function. Namely:
 - BootNext
 - OsIndications
 - LoaderConfigConsoleMode
 - LoaderEntryLastBooted

Of which the latter two are systemd specific, even though they are
undocumented. Ensure they're removed - follow-up commits will add
documentation references.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>