mxmount: Remove unused variables

DEFAULT_MOUNT_FS,FULLHOST,NONE,NULL are never used. Remove unused
variables.

Note, that SHORTHOST is currently used by /etc/mxmounts.

mxmount/mxmount

@@ -27,16 +27,11 @@ $fullhostname = hostname();
 
 %D = ();
 %V = (
-    DEFAULT_MOUNT_FS       => '',
     DEFAULT_MOUNT_OPTIONS  => '',
     DEFAULT_EXPORT_OPTIONS => '',
     DEFAULT_MOUNT_PREFIX   => '/',
 
     SHORTHOST => $hostname,
-    FULLHOST  => $fullhostname,
-
-    NONE => '',
-    NULL => ''
 );
 
 @lines   = read_file($configfile);