Fix imgtool encrypt rebase issue

Avoid passing invalid `encrypt` param to image constructor.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool.py b/scripts/imgtool.py
index 61122b4..b0ff189 100755
--- a/scripts/imgtool.py
+++ b/scripts/imgtool.py
@@ -141,8 +141,7 @@
                            pad=pad, align=int(align), slot_size=slot_size,
                            max_sectors=max_sectors,
                            overwrite_only=overwrite_only,
-                           endian=endian,
-                           encrypt=encrypt)
+                           endian=endian)
     key = load_key(key) if key else None
     enckey = load_key(encrypt) if encrypt else None
     if enckey: