wic: remove duplicate --boot from x-gpt-efi-disk

The x-gpt-efi-disk kickstart file has a duplicate --boot argument for
the boot partition. One uses the BOOT_PARTITION_LABEL variable. The other
uses a hard-coded string "boot". Drop the hard-coded value.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>