board/zynqmp/kria/kv260/uboot.fragment: remove unnecessary CONFIG_MULTI_DTB_FIT option

This patch removes the CONFIG_MULTI_DTB_FIT u-boot option for the
zynqmp_kria_kv260_defconfig as it is not necessary.  The post build
kv260.sh creates the proper u-boot.itb without needing this option.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3310463281



Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>