Remove src/llvm-emscripten submodule

With #65251 landed there's no need to build two LLVM backends and ship
them with rustc, every target we have now uses the same LLVM backend!

This removes the `src/llvm-emscripten` submodule and additionally
removes all support from rustbuild for building the emscripten LLVM
backend. Multiple codegen backend support is left in place for now, and
this is intended to be an easy 10-15 minute win on CI times by avoiding
having to build LLVM twice.

src/bootstrap/lib.rs

@@ -232,7 +232,6 @@ pub struct Build {
     miri_info: channel::GitInfo,
     rustfmt_info: channel::GitInfo,
     in_tree_llvm_info: channel::GitInfo,
-    emscripten_llvm_info: channel::GitInfo,
     local_rebuild: bool,
     fail_fast: bool,
     doc_tests: DocTests,
@@ -351,7 +350,6 @@ impl Build {
 
         // we always try to use git for LLVM builds
         let in_tree_llvm_info = channel::GitInfo::new(false, &src.join("src/llvm-project"));
-        let emscripten_llvm_info = channel::GitInfo::new(false, &src.join("src/llvm-emscripten"));
 
         let mut build = Build {
             initial_rustc: config.initial_rustc.clone(),
@@ -376,7 +374,6 @@ impl Build {
             miri_info,
             rustfmt_info,
             in_tree_llvm_info,
-            emscripten_llvm_info,
             cc: HashMap::new(),
             cxx: HashMap::new(),
             ar: HashMap::new(),
@@ -553,10 +550,6 @@ impl Build {
         self.out.join(&*target).join("llvm")
     }
 
-    fn emscripten_llvm_out(&self, target: Interned<String>) -> PathBuf {
-        self.out.join(&*target).join("llvm-emscripten")
-    }
-
     fn lld_out(&self, target: Interned<String>) -> PathBuf {
         self.out.join(&*target).join("lld")
     }