[spirv] Remove OpKill helper

This is deprecated, so make sure we never accidentally
use it in the future.

src/spirv/spirv_module.cpp

@@ -3666,12 +3666,6 @@ namespace dxvk {
   }
   
   
-  void SpirvModule::opKill() {
-    m_code.putIns (spv::OpKill, 1);
-    m_blockId = 0;
-  }
-  
-  
   void SpirvModule::opDemoteToHelperInvocation() {
     m_code.putIns (spv::OpDemoteToHelperInvocation, 1);
   }

src/spirv/spirv_module.h

@@ -1252,8 +1252,6 @@ namespace dxvk {
     
     void opReturn();
     
-    void opKill();
-
     void opDemoteToHelperInvocation();
     
     void opEmitVertex(