Remove invalid variable usage

There is no variable called "startup" in awful.rules.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/rules.lua.in

@@ -127,7 +127,7 @@ function apply(c)
 
     -- If untagged, stick the client on the current one.
     if #c:tags() == 0 then
-        atag.withcurrent(c, startup)
+        atag.withcurrent(c)
     end
 
     -- Apply all callbacks from matched rules.