Remove SPIRVDawnCompatFlag from SkSL.

In practice this was only used for synthesizing separate texture
sampler pairs. We can get the same information with layout flags
instead, allowing us to remove the setting.

This code can be removed entirely once we've fully transitioned
to the WGSL backend for WebGPU and stop relying on the SPIR-V
reader.

Change-Id: I18f5ccb707a9e4d42fa85a68bd13a3139d729f33
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/749141
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
16 files changed