Remove SPIR-V option from Graphite Dawn backend.

Now that the WGSL backend is enabled and stable, we no longer need
support for the SPIR-V reader compatibility path for fragment and
vertex shaders.

Change-Id: I95621567ad259fec730f8d2a2aea6ea502a09e11
Bug: b/40044196
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/752898
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
9 files changed