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/build/expr/as_rvalue.rs

@@ -250,7 +250,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Deref { .. }
             | ExprKind::Index { .. }
             | ExprKind::VarRef { .. }
-            | ExprKind::SelfRef
             | ExprKind::UpvarRef { .. }
             | ExprKind::Break { .. }
             | ExprKind::Continue { .. }