Remove MirSource::from_instance helper that is only used in one place

It used to be much more prevalent before I extracted shims as their own enum.

Author: camelid

compiler/rustc_middle/src/mir/mod.rs

@@ -127,10 +127,6 @@ impl<'tcx> MirSource<'tcx> {
         MirSource { instance: InstanceKind::Item(def_id), promoted: None }
     }
 
-    pub fn from_instance(instance: InstanceKind<'tcx>) -> Self {
-        MirSource { instance, promoted: None }
-    }
-
     pub fn from_shim(shim: ShimKind<'tcx>) -> Self {
         MirSource { instance: InstanceKind::Shim(shim), promoted: None }
     }

compiler/rustc_mir_transform/src/coroutine/by_move_body.rs

@@ -223,8 +223,10 @@ pub(crate) fn coroutine_by_move_body_def_id<'tcx>(
         None,
         &mut PerParentDisambiguatorState::new(parent_def_id),
     );
-    by_move_body.source =
-        mir::MirSource::from_instance(InstanceKind::Item(body_def.def_id().to_def_id()));
+    by_move_body.source = mir::MirSource {
+        instance: InstanceKind::Item(body_def.def_id().to_def_id()),
+        promoted: None,
+    };
 
     if let Some(dumper) = MirDumper::new(tcx, "built", &by_move_body) {
         dumper.set_disambiguator(&"after").dump_mir(&by_move_body);