Revert "ar71xx: Add ubi/ubifs to kernel config."

This reverts commit 87f6f40f709e5b160a4a283193de50fc1374f6ed.
ar71xx: Create netgear subtarget and move wndr4300 there

This commit creates a netgear subtarget, and moves the wndr4300
profile into it. Furthermore, the FEATURES are moved to the
subtarget definition. The subtarget is UBI(FS) only, so all
SquashFS/JFFS2 defines are removed in its config-default.

The image Makefile has some small adjustments to ensure the
proper routines get called now that the profile is in a
different subtarget.