Remove hooking of system color changes (dotnet/corefx#39468)

Remove hooking of system color changes (dotnet/corefx#39468)

* Remove hooking of system color changes

Hooking user preference changes to update system colors is expensive, particularly as it is hooked via reflection. Getting the current color from the OS is extremely cheap.  Removing this layer of caching avoids creating a watcher window and thread and a significant number of startup allocations.

In starting up a blank WinForms app this cuts 200 allocations.

Note that Pens and Brushes in System.Drawing start their own user preference watcher so that they can update GDI+ when backed by system colors.

* Address feedback

* Remove newly dead files

Commit migrated from https://github.com/dotnet/corefx/commit/97c736e09f2e119d6ec3d3ebebb7234dee48145d