Move SPIRV-Cross functions into OpenGL backend.

Since the other backends no longer need it, move all of the
utility functions specific to SPIRV-Cross into the OpenGL backend.
This obviates the need for the DAWN_USE_SPIRV_CROSS define, so remove it.

Bug: dawn:1036

Change-Id: I67bb5a85dc128a6f343d09876046cf559395e05f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/61541
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/common/BUILD.gn

@@ -85,10 +85,6 @@ config("dawn_internal") {
     defines += [ "DAWN_USE_X11" ]
   }
 
-  if (dawn_use_spirv_cross) {
-    defines += [ "DAWN_USE_SPIRV_CROSS" ]
-  }
-
   if (dawn_enable_error_injection) {
     defines += [ "DAWN_ENABLE_ERROR_INJECTION" ]
   }