Remove unnecessary forward declare of GrVkExtensions

This is unnecessary, since we already include the file that declares it.
Furthermore, https://skia-review.googlesource.com/c/skia/+/571497
renames the class and makes the old name a `using` statement. We are
currently unable to merge that CL because it breaks the forward
declaration. Remove it so that Skia can move forward.

Bug: NA
Test: make
Change-Id: I0a7cda031f61ba01d48a5a8568ebeffd955b1bc7