Revert "image-fit: Fix signature checking"

This reverts commit 84ca65aa.

On signature verification failures fit_image_verify() should NOT exit with
error. Only keys marked 'required' can cause image verification failure.
This logic is already there and works correctly.

Add a comment to make this clear.

Fixes: 84ca65aa (image-fit: Fix signature checking)
Signed-off-by: Simon Glass <sjg@chromium.org>