Deprecate no-op codegen option -Cinline-threshold=...

This deprecates `-Cinline-threshold` since using it has no effect. This
has been the case since the new LLVM pass manager started being used,
more than 2 years ago.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -564,9 +564,6 @@ pub(crate) unsafe fn llvm_optimize(
 
     let llvm_plugins = config.llvm_plugins.join(",");
 
-    // FIXME: NewPM doesn't provide a facility to pass custom InlineParams.
-    // We would have to add upstream support for this first, before we can support
-    // config.inline_threshold and our more aggressive default thresholds.
     let result = llvm::LLVMRustOptimize(
         module.module_llvm.llmod(),
         &*module.module_llvm.tm,