Remove THIR::ExprKind::SelfRef

ExprKind::SelfRef was used to express accessing `self` in
the desugared Closure/Generator struct when lowering captures in THIR.

Since we handle captures in MIR now, we don't need `ExprKind::Self`.

compiler/rustc_mir_build/src/thir/mod.rs

@@ -219,8 +219,6 @@ crate enum ExprKind<'tcx> {
         /// HirId of the root variable
         var_hir_id: hir::HirId,
     },
-    /// first argument, used for self in a closure
-    SelfRef,
     Borrow {
         borrow_kind: BorrowKind,
         arg: ExprRef<'tcx>,