lib: rsa: Remove #ifdefs from rsa.h

It is no longer necessary to implement rsa_() functions as no-ops
depending on config options. It is merely sufficient to provide the
prototypes, as the rsa code is no longer linked when unused.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>