Fixes commit 1df078158f ("doc: build: Add signing page") that removed
the `imgtool` section from the `sign.rst` page.
rimage configuration was designed in a very different way from imgtool
configuration. The rimage doc section was following the imgtool doc
section and constrating the two approaches. Now that the previous
imgtool doc section has just been removed, some words like "different"
and "instead" don't make sense anymore. Drop them.
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
fe9bfe5335