doc: Remove information about CAPSULE_FMP_HEADER

This Kconfig symbol was never added to U-Boot but it was mentioned in the
origin commit c35df7c9 ("qemu: arm64: Add documentation for capsule
update"). That's why remove it from documentation to be accurate.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>