config: enable shadow passwords unconditionally

Configurations without shadow passwords have been broken since the removal
of telnet: as the default entry in /etc/passwd is not empty (but rather
unset), there will be no way to log onto such a system by default. As
disabling shadow passwords is not useful anyways, remove this configuration
option.

The config symbol is kept (for a while), as packages from feeds depend on
it.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>