Remove sourcing /scripts/functions in components

The components 9990-fstab.sh and 9990-netbase.sh source
/scripts/functions. This overrides the mount_* functions (to no-ops)
which will break supporting live-{top,premount,bottom} hooks. Therefore
remove those problematic includes which do not seem to be necessary
nowadays. See Debian bug #884355 for the discussion.

Signed-off-by: Benjamin Drung <benjamin.drung@profitbricks.com>