Remove unreachable default clauses from dwarf.dart.

I'm currently working on a fix to the analyzer that will cause
unreachable default clauses to be properly flagged as a warning
(https://github.com/dart-lang/sdk/issues/54575). To avoid a bot
failure when the fix lands, this unreachable default clause needs to
be removed.

Bug: https://github.com/dart-lang/sdk/issues/54575
Change-Id: I8b44341fad3146d4fdd64831e263513fd5092102
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/380075
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

pkg/native_stack_traces/lib/src/dwarf.dart

@@ -2130,8 +2130,6 @@ class DwarfSnapshot extends Dwarf {
           throw 'Cannot locate isolate instructions section in snapshot';
         }
         return pcOffset.offset + isolateStart;
-      default:
-        throw 'Unexpected value for instructions section';
     }
   }