vncserver: remove securitytypes mention from man page

securitytypes is no longer used.

unix/vncserver.man

@@ -171,9 +171,9 @@ $HOME/.vnc/config
 An optional server config file wherein options to be passed to Xvnc are listed
 to avoid hard-coding them to the physical invocation. List options in this file
 one per line. For those requiring an argument, simply separate the option from
-the argument with an equal sign, for example: "geometry=2000x1200" or
-"securitytypes=vncauth,tlsvnc". Options without an argument are simply listed
-as a single word, for example: "localhost" or "alwaysshared".
+the argument with an equal sign, for example: "geometry=2000x1200". Options
+without an argument are simply listed as a single word, for example: "localhost"
+or "alwaysshared".
 .TP
 $HOME/.vnc/passwd
 The VNC password file.