Remove the leftover geometry shader scaffolding

Nothing has generated or used a geometry shader since the GS paths were removed
- GeometryShaderGenerator is gone, and ShaderWriter's BeginGSMain/EndGSMain had
no callers at all. Removes ShaderStage::Geometry and everything hanging off it:
the GS preambles and GSMain helpers in ShaderWriter, the stage mappings in all
three thin3d backends, the D3D11 geometry shader plumbing (curGS_, the pipeline
and module members, gs_4_0 compilation), CreateGeometryShaderD3D11, the unused
PipelineFlags::USES_GEOMETRY_SHADER and PipelineManagerVulkan's
UsesGeometryShader().

Also stop enabling the Vulkan geometryShader device feature, since we no longer
have any use for it.

Kept on purpose: the device feature is still listed in the feature dumps (like
other capabilities we don't use), and the Vulkan shader cache header keeps its
now-always-zero geometry shader count so the on-disk format stays compatible.

Common/GPU/Vulkan/VulkanContext.cpp

@@ -822,7 +822,6 @@ VkResult VulkanContext::CreateDevice(int physical_device, const std::vector<cons
 	deviceFeatures_.enabled.standard.samplerAnisotropy = deviceFeatures_.available.standard.samplerAnisotropy;
 	deviceFeatures_.enabled.standard.shaderClipDistance = deviceFeatures_.available.standard.shaderClipDistance;
 	deviceFeatures_.enabled.standard.shaderCullDistance = deviceFeatures_.available.standard.shaderCullDistance;
-	deviceFeatures_.enabled.standard.geometryShader = deviceFeatures_.available.standard.geometryShader;
 	deviceFeatures_.enabled.standard.sampleRateShading = deviceFeatures_.available.standard.sampleRateShading;
 
 	// A host application (e.g. a libretro frontend) may require some additional features to be enabled.