Remove unused first parameter in some constructors

Fixes #3214.

Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>

lib/wibox/container/place.lua

@@ -199,8 +199,8 @@ local function new(widget, halign, valign)
     return ret
 end
 
-function place.mt:__call(_, ...)
-    return new(_, ...)
+function place.mt:__call(...)
+    return new(...)
 end
 
 --@DOC_widget_COMMON@

lib/wibox/container/radialprogressbar.lua

@@ -277,7 +277,7 @@ local function new(widget)
     return ret
 end
 
-function radialprogressbar.mt:__call(_, ...)
+function radialprogressbar.mt:__call(...)
     return new(...)
 end
 

lib/wibox/layout/stack.lua

@@ -204,7 +204,7 @@ local function new(...)
     return ret
 end
 
-function stack.mt:__call(_, ...)
+function stack.mt:__call(...)
     return new(...)
 end
 

lib/wibox/widget/slider.lua

@@ -534,7 +534,7 @@ local function new(args)
     return ret
 end
 
-function slider.mt:__call(_, ...)
+function slider.mt:__call(...)
     return new(...)
 end