Remove leftover from emscripten fastcomp support

This is no longer used since rust-lang#63649

Author: bjorn3

src/librustc_interface/util.rs

@@ -235,13 +235,8 @@ pub fn get_codegen_backend(sess: &Session) -> Box<dyn CodegenBackend> {
     static mut LOAD: fn() -> Box<dyn CodegenBackend> = || unreachable!();
 
     INIT.call_once(|| {
-        let codegen_name = sess
-            .opts
-            .debugging_opts
-            .codegen_backend
-            .as_ref()
-            .unwrap_or(&sess.target.target.options.codegen_backend);
-        let backend = match &codegen_name[..] {
+        let codegen_name = sess.opts.debugging_opts.codegen_backend.as_deref().unwrap_or("llvm");
+        let backend = match codegen_name {
             filename if filename.contains('.') => load_backend_from_dylib(filename.as_ref()),
             codegen_name => get_builtin_codegen_backend(codegen_name),
         };

src/librustc_target/spec/mod.rs

@@ -938,9 +938,6 @@ pub struct TargetOptions {
     /// for this target unconditionally.
     pub no_builtins: bool,
 
-    /// The codegen backend to use for this target, typically "llvm"
-    pub codegen_backend: String,
-
     /// The default visibility for symbols in this target should be "hidden"
     /// rather than "default"
     pub default_hidden_visibility: bool,
@@ -1068,7 +1065,6 @@ impl Default for TargetOptions {
             requires_lto: false,
             singlethread: false,
             no_builtins: false,
-            codegen_backend: "llvm".to_string(),
             default_hidden_visibility: false,
             emit_debug_gdb_scripts: true,
             requires_uwtable: false,
@@ -1461,7 +1457,6 @@ impl Target {
         key!(requires_lto, bool);
         key!(singlethread, bool);
         key!(no_builtins, bool);
-        key!(codegen_backend);
         key!(default_hidden_visibility, bool);
         key!(emit_debug_gdb_scripts, bool);
         key!(requires_uwtable, bool);
@@ -1699,7 +1694,6 @@ impl ToJson for Target {
         target_option_val!(requires_lto);
         target_option_val!(singlethread);
         target_option_val!(no_builtins);
-        target_option_val!(codegen_backend);
         target_option_val!(default_hidden_visibility);
         target_option_val!(emit_debug_gdb_scripts);
         target_option_val!(requires_uwtable);