Remove unused ci color kernel, previously used for grayscale filter

SilicaViewer/Document.swift

@@ -872,39 +872,6 @@ public extension NSImage {
     }
 }
 
-class ColorFilter: CIFilter {
-  // 2
-  var inputImage: CIImage?
-
-  // 3
-  static var kernel: CIKernel = { () -> CIColorKernel in
-    guard let url = Bundle.main.url(forResource: "default",
-      withExtension: "metallib"),
-      let data = try? Data(contentsOf: url) else {
-      fatalError("Unable to load metallib")
-    }
-
-    guard let kernel = try? CIColorKernel(
-      functionName: "colorFilterKernel",
-      fromMetalLibraryData: data) else {
-      fatalError("Unable to create color kernel")
-    }
-
-    return kernel
-  }()
-
-  // 4
-  override var outputImage: CIImage? {
-    guard let inputImage = inputImage else { return nil }
-    return ColorFilter.kernel.apply(
-      extent: inputImage.extent,
-      roiCallback: { _, rect in
-        return rect
-      },
-      arguments: [inputImage])
-  }
-}
-
 extension CGImage {
     func toGrayscale() -> CGImage? {
         let rect = CGRect(x: 0, y: 0, width: width, height: height)