xrandr.lua: Remove useless use of unpack()

This code would have to check if unpack or table.unpack is the function
to use. Instead of adding this check, this commit just removes the
trivial use of unpack.

Fixes: https://github.com/awesomeWM/awesome/issues/2802
Signed-off-by: Uli Schlachter <psychon@znc.in>

recipes/xrandr.lua

@@ -118,7 +118,7 @@ local function xrandr()
       label = "Keep the current configuration"
       state.index = nil
    else
-      label, action = unpack(next)
+      label, action = next[1], next[2]
    end
    state.cid = naughty.notify({ text = label,
                                 icon = icon_path,