remove superfluous skia_vulkan_sdk

We don't actually need the SDK to build anymore.

If you previously set skia_vulkan_sdk or relied on VULKAN_SDK
in the environment, now set skia_use_vulkan = true instead.

We still need the linux_vulkan_sdk asset for Test/Perf bots.

Change-Id: I70ad9c5181ae61725b9dc25cc887dd0640beeed6
Reviewed-on: https://skia-review.googlesource.com/c/184065
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 60158c7..c34d99a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -79,11 +79,8 @@
     skia_use_vulkan = defined(ndk_api) && ndk_api >= 24
   } else if (is_fuchsia) {
     skia_use_vulkan = fuchsia_use_vulkan
-  } else if (is_win && target_cpu == "arm64") {
-    skia_use_vulkan = false
   } else {
-    skia_use_vulkan = (defined(skia_vulkan_sdk) && skia_vulkan_sdk != "") ||
-                      (defined(skia_moltenvk_path) && skia_moltenvk_path != "")
+    skia_use_vulkan = defined(skia_moltenvk_path) && skia_moltenvk_path != ""
   }
 
   if (is_ios) {