[package] base-files: remove network preconfig, it collides with ar71xx network first...

[package] base-files: remove network preconfig, it collides with ar71xx network firstboot uci-defaults
To supply custom network settings, it is better to embed a complete network config file using the
files/ overlay, the old image lan preconfig options are also very limited in scope and actually collide
with sysupgrade and firstboot hardware detection on at least ar71xx and possibly other targets, therfore
drop this facility.

SVN-Revision: 31258