Remove the abi_amdgpu_kernel feature

compiler/rustc_codegen_llvm/src/abi.rs

@@ -590,7 +590,6 @@ impl From<Conv> for llvm::CallConv {
             Conv::Cold => llvm::ColdCallConv,
             Conv::PreserveMost => llvm::PreserveMost,
             Conv::PreserveAll => llvm::PreserveAll,
-            Conv::AmdGpuKernel => llvm::AmdGpuKernel,
             Conv::AvrInterrupt => llvm::AvrInterrupt,
             Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,
             Conv::ArmAapcs => llvm::ArmAapcsCallConv,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -106,7 +106,6 @@ pub enum CallConv {
     X86_Intr = 83,
     AvrNonBlockingInterrupt = 84,
     AvrInterrupt = 85,
-    AmdGpuKernel = 91,
 }
 
 /// LLVMRustLinkage