Don't set the background color when initializing with transparency (#3657)

Setting the background color changes how the window title bar appears,
which is something that the application should customize itself if it
wants this behaviour (and also, it wasn't set when calling
`set_transparent`, so the behaviour wasn't consistent).

Cargo.toml

@@ -137,7 +137,6 @@ features = [
     "NSApplication",
     "NSBitmapImageRep",
     "NSButton",
-    "NSColor",
     "NSControl",
     "NSCursor",
     "NSDragging",