Remove experimental continuation support (#2776)

Removes `__continuation`, `__await`, and `__run`.

In part here, the discussion was that while the feature had been useful for validating the early explorer design, it's no longer needed for that role as the explorer is now quite robust. Continuations have been experimental and, at this point, don't have an owner pushing to a proposal.

The triggering factor is that, as we push to address fuzzer issues, I ran into a crash bug in this code; basically, `fn Main() -> i32 { __await; return 0; }`. When I mentioned this, the reaction seemed to trend towards removal of the feature.

explorer/ast/statement.cpp

@@ -123,24 +123,6 @@ void Statement::PrintDepth(int depth, llvm::raw_ostream& out) const {
       }
       break;
     }
-    case StatementKind::Continuation: {
-      const auto& cont = cast<Continuation>(*this);
-      out << "continuation " << cont.name() << " ";
-      if (depth < 0 || depth > 1) {
-        out << "\n";
-      }
-      cont.body().PrintDepth(depth - 1, out);
-      if (depth < 0 || depth > 1) {
-        out << "\n";
-      }
-      break;
-    }
-    case StatementKind::Run:
-      out << "run " << cast<Run>(*this).argument() << ";";
-      break;
-    case StatementKind::Await:
-      out << "await;";
-      break;
   }
 }