app: remove unused types and func arguments

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

app/os.go

@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// package app implements platform specific windows
-// and GPU contexts.
 package app
 
 import (
@@ -17,11 +15,6 @@ import (
 	"gioui.org/unit"
 )
 
-type size struct {
-	Width  unit.Value
-	Height unit.Value
-}
-
 // errOutOfDate is reported when the GPU surface dimensions or properties no
 // longer match the window.
 var errOutOfDate = errors.New("app: GPU surface out of date")