Remove special handling of the USERNAME environment variable. It

used to be set on old versions of Fedora but that hasn't been the
case for some time.  It's worth noting that ssh doesn't set USERNAME
either.

plugins/sudoers/logging.c

@@ -637,8 +637,10 @@ send_mail(const char *fmt, ...)
 	"HOME=/",
 	"PATH=/usr/bin:/bin:/usr/sbin:/sbin",
 	"LOGNAME=root",
-	"USERNAME=root",
 	"USER=root",
+# ifdef _AIX
+	"LOGIN=root",
+# endif
 	NULL
     };
 #endif /* NO_ROOT_MAILER */