Drop sbsigntools

Let's prefer using systemd-sbsign from systemd instead.

Author: DaanDeMeyer

mkosi.conf.d/20-arch/mkosi.conf

@@ -25,7 +25,6 @@ Packages=
         psmisc
         python3
         qrencode
-        sbsigntools
         shadow
         systemd-ukify
         tgt

mkosi.conf.d/20-fedora/mkosi.conf

@@ -30,7 +30,6 @@ Packages=
         procps-ng
         python3
         rpm
-        sbsigntools
         systemd-boot
         systemd-ukify
         tpm2-tools