Replace `unimpl` comments with `= delete`.

A handful of `unimpl`ed copy constructors were actually supplied as
{} instead of being left unimplemented, but they were not actually used.

Change-Id: I9b88173aeb0cc87f793ee40dfba1b5e5a06d7497
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308565
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/src/gpu/GrImageContextPriv.h b/src/gpu/GrImageContextPriv.h
index ea7c740..aa7c4c3 100644
--- a/src/gpu/GrImageContextPriv.h
+++ b/src/gpu/GrImageContextPriv.h
@@ -39,8 +39,8 @@
 
 private:
     explicit GrImageContextPriv(GrImageContext* context) : fContext(context) {}
-    GrImageContextPriv(const GrImageContextPriv&); // unimpl
-    GrImageContextPriv& operator=(const GrImageContextPriv&); // unimpl
+    GrImageContextPriv(const GrImageContextPriv&) = delete;
+    GrImageContextPriv& operator=(const GrImageContextPriv&) = delete;
 
     // No taking addresses of this type.
     const GrImageContextPriv* operator&() const;