bootstage: Remove unused entries related to kernel/ramdisk/fdt load

Now that the code for loading these three images from a FIT is common, we
don't need individual boostage IDs for each of them.

Note: there are some minor changes in the bootstage numbering, particuarly
for kernel loading. I don't believe this matters.

Signed-off-by: Simon Glass <sjg@chromium.org>