base-files: do not source system.sh in functions.sh

base-files: do not source system.sh in functions.sh

The default_postinst() function in /lib/functions.sh sources
/lib/functions/system.sh before cycling through uci-defaults files.

This creates a pseudo-cyclic dependency as system.sh also uses
functions that are located in functions.sh. Despite that, there
is actually only one uci-defaults file in the entire repo that needs
system.sh, and this one contains an explicit source for system.sh
anyway.

Consequently, this patch removes the sourcing of system.sh in
functions.sh. There are no relevant uses in packages, routing and
luci repositories.
This may require adjustments for downstream, though.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>