Do not use tabbed by default

With transition to tiled window managers, tabbed makes less sense.

dotfiles.config/Xlib/global.Xdefaults

 URxvt.mapAlert:             true
 
 !## extensions
-URxvt.perl-ext-common:      default,tabbed,selection
+URxvt.perl-ext-common:      default,selection
 URxvt.tabbed.tab-fg:        7
 URxvt.tabbed.tab-bg:        0
 URxvt.tabbed.tabbar-fg:     4