imgtool: Remove default padding of ECDSA sigs

Since commit:

    commit a3608266
    Author: David Brown <david.brown@linaro.org>
    Date:   Thu Dec 12 15:35:31 2019 -0700

        ecdsa: Allow ECDSA signatures to be actual length

MCUboot no longer requires ECDSA signatures to be padded to a fixed
length.  This change makes imgtool, by default, also no longer add this
padding to images.  There is an option `--pad-sig` that can be given to
the sign command to re-instate this padding.  This flag will be needed
to make images that will work with older (pre 1.5.0) versions of
MCUboot.

Signed-off-by: David Brown <david.brown@linaro.org>