remove togglefullscreen keybinding

Distribute it as a patch like in dwm since graphical applications
usually provide their own keybinding; I guess it's only for terminals.

Note that even though these commits don't let you open multiple windows
in fullscreen and cycle between them like in dwm, with just
fullscreennotify spawning new windows or changing tag would still exit
fullscreen automatically, but you would have to toggle fullscreen twice
when switching back to the fullscreen window to enter fullscreen again,
so this is better since it avoids that.

config.def.h

@@ -75,7 +75,6 @@ static const Key keys[] = {
 	{ MODKEY,                    XKB_KEY_m,          setlayout,      {.v = &layouts[2]} },
 	{ MODKEY,                    XKB_KEY_space,      setlayout,      {0} },
 	{ MODKEY|WLR_MODIFIER_SHIFT, XKB_KEY_space,      togglefloating, {0} },
-	{ MODKEY, 					 XKB_KEY_e,    		togglefullscreen, {0} },
 	{ MODKEY,                    XKB_KEY_0,          view,           {.ui = ~0} },
 	{ MODKEY|WLR_MODIFIER_SHIFT, XKB_KEY_parenright, tag,            {.ui = ~0} },
 	{ MODKEY,                    XKB_KEY_comma,      focusmon,       {.i = -1} },