refactor(buck2_transition): remove use of try_blocks [rustc-stable]

Summary: Unnecessary. The only use case was actually a bit roundabout, from
what I can tell: it was used to unwrap() a SharedResult using `?`, only so it
could immediately be re-packaged as a SharedResult from the result of the 'try'
expression.

Just skip all that, and let the value be returned directly.

GitHub Issue: facebook#265

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: Ittluzprupvzmuprmspuzqnzuxovoxuyn

app/buck2_transition/src/lib.rs

@@ -7,8 +7,6 @@
  * of this source tree.
  */
 
-#![feature(try_blocks)]
-
 pub(crate) mod coerced_attr;
 pub(crate) mod transition;
 

app/buck2_transition/src/transition/calculation_apply_transition.rs

@@ -284,10 +284,9 @@ impl TransitionCalculation for TransitionCalculationImpl {
                 ctx: &DiceComputations,
                 _cancellation: &CancellationContext,
             ) -> Self::Value {
-                let v: SharedResult<_> = try {
+                let v: SharedResult<_> =
                     do_apply_transition(ctx, self.attrs.as_deref(), &self.cfg, &self.transition_id)
-                        .await?
-                };
+                        .await;
 
                 Ok(Arc::new(v.with_context(|| {
                     format!("Error computing transition `{}`", self)