utils: get rid of mu-util.[ch]

It was getting old...
Remove the unused parts, refactor the few parts still in use.

lib/tests/test-mu-maildir.cc

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2008-2022 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2008-2023 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -29,7 +29,6 @@
 #include "utils/mu-test-utils.hh"
 #include "mu-maildir.hh"
 #include "utils/mu-result.hh"
-#include "utils/mu-util.h"
 
 using namespace Mu;
 

lib/tests/test-mu-store.cc

@@ -33,6 +33,7 @@
 #include "mu-store.hh"
 #include "utils/mu-result.hh"
 #include <utils/mu-utils.hh>
+#include <utils/mu-utils-file.hh>
 #include "mu-maildir.hh"
 
 using namespace Mu;