mkosi: update Debian config for mkosi (systemd#6089)

Drop g++ package — it's only used for a test and we want to keep the mkosi installation footprint small-ish.

Author: AsciiWolf

.mkosi/mkosi.debian

@@ -34,7 +34,6 @@ BuildPackages=
         acl
         docbook-xml
         docbook-xsl
-        g++
         gcc
         git
         gnu-efi