x86: Simplify cpu_jump_to_64bit_uboot()

This copies the cpu_call64() function to memory address and then jumps to
it. This seems to work correctly even when called from SPL, which is
running from SPI flash.

Drop the copy as it is not needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>