doc: Remove README.sha1 file

The contents of README.sha1 only refer to process around verification
of the pcs440ep board firmware in flash. The device was removed in
commit 242836a8 ("powerpc: ppc4xx: remove pcs440ep support") in
2015 so this readme isn't really relevant anymore so can be removed.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>