Dropped Support for Bidirectional Custom Target Definition Emulation

as requested in the review and argued that this is only consistent with later LLVM upgrades

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -144,17 +144,6 @@ fn set_probestack(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
     );
 }
 
-fn translate_obsolete_target_features(feature: &str) -> &str {
-    const LLVM9_FEATURE_CHANGES: &[(&str, &str)] =
-        &[("+fp-only-sp", "-fp64"), ("-fp-only-sp", "+fp64"), ("+d16", "-d32"), ("-d16", "+d32")];
-    for &(old, new) in LLVM9_FEATURE_CHANGES {
-        if feature == old {
-            return new;
-        }
-    }
-    feature
-}
-
 pub fn llvm_target_features(sess: &Session) -> impl Iterator<Item = &str> {
     const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
 
@@ -169,7 +158,6 @@ pub fn llvm_target_features(sess: &Session) -> impl Iterator<Item = &str> {
         .split(',')
         .chain(cmdline)
         .filter(|l| !l.is_empty())
-        .map(translate_obsolete_target_features)
 }
 
 pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {