Revert "infra: document support for booting on AArch64 hosts"

This reverts commit 0e3935f2. The build
and boot on a AArch64 is now support (for busybox boot). Hence, the
existing procedure of building the stack on x86 and executing on AArch64
host is now not required.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>