ARM: shmobile: defconfig: Drop NET_VENDOR_<FOO>=n

Enabling NET_VENDOR_* Kconfig options does not directly affect the
kernel, so there is no need to explicitly disable them.
The individual network drivers under them are still disabled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>