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/thin3d_vulkan.cpp

@@ -174,7 +174,6 @@ public:
 VkShaderStageFlagBits StageToVulkan(ShaderStage stage) {
 	switch (stage) {
 	case ShaderStage::Vertex: return VK_SHADER_STAGE_VERTEX_BIT;
-	case ShaderStage::Geometry: return VK_SHADER_STAGE_GEOMETRY_BIT;
 	case ShaderStage::Compute: return VK_SHADER_STAGE_COMPUTE_BIT;
 	case ShaderStage::Fragment: return VK_SHADER_STAGE_FRAGMENT_BIT;
 	}