mailutils: update guile to current version (#103209)

Apparently a previous version of mailutils didn't compile
with guile 2.2, but this does no longer appear to be the case.

This is particularly relevant because mailutils is part of
the iso_minimal test set we are currently focusing on for
getting more of NixOS reproducible, and guile_2_0 is not
reproducible (https://r13y.com/)

pkgs/top-level/all-packages.nix

@@ -5524,7 +5524,6 @@ in
   mailpile = callPackage ../applications/networking/mailreaders/mailpile { };
 
   mailutils = callPackage ../tools/networking/mailutils {
-    guile = guile_2_0;  # compilation fails with guile 2.2
     sasl = gsasl;
   };