docs: Remove redundant documentation about combined payload use case

U-Boot now supports loading Linux kernel image via network and storage
media. Thus, we don't need to use a combined payload containing both
U-Boot & Linux kernel image to boot Linux from U-Boot prompt.

Remove the old documentation.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>