sunxi: Remove CONFIG_TARGET_FOO for sun5i and sun7i boards

CONFIG_TARGET_FOO was only used in board/sunxi/Makefile to select the
dram config for sun5i and sun7i boards and in board/sunxi/gmac.c for some
special handling of the bananapi/bananapro (both sun7i), all sun5i and sun7i
boards have been moved over to using a single dram_sun5i_autoconfig file,
and the tx clk delay handling for the Banana boards now has its own Kconfig.

IOW nothing is using CONFIG_TARGET_FOO anymore, so remove it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>