sign.sh: stop telling gpg to load the 'idea' extension.

The IDEA symmetric cipher was the standard one used to protect trad
PGP private keys, back in the days when PuTTY had its very first set.
We haven't needed this option for a long time, but it didn't cause any
obvious failures, so I never spotted it and removed it from the build
script.

But it does cause a failure now, because gpg on Ubuntu 24.04 reports
'invalid option "--load-extension=idea"', suggesting that it hasn't
just forgotten about _that_ extension, it doesn't even like extensions
at all any more. Happily, we don't need it.

sign.sh

@@ -38,7 +38,7 @@ sign() {
   # through.
   echo "----- Signing $2 with key '$keyname'"
   test -f "$3" || \
-    gpg --load-extension=idea "$1" -u "$keyname" -o "$3" "$2"
+    gpg "$1" -u "$keyname" -o "$3" "$2"
 }
 
 cd "$1"