sunxi: drop default SPL_LIBDISK_SUPPORT enablement

sunxi: drop default SPL_LIBDISK_SUPPORT enablement

There is no code for using partition labels in the Allwinner SPL port.
Even so the name is slightly misleading, CONFIG_SPL_LIBDISK_SUPPORT was
meant to guard partition code for the SPL.

Remove the "imply" line in the Kconfig to make this obvious and avoid
unneeded code inclusions, helping to keep the H6 SPL code small.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>