[EventEngine] rm invalid assert (#35953)

In the case where the timer delay is <= 0, RunAfter will return an invalid handle since the callback will not be cancellable. The asserting is invalid in this case. Based on how this handle is used, it seems there is no need for a check that the handle is set. It would only matter if `finish_bdp_ping` were called again before `next_bdp_ping_timer_expired` were called, which I don't think is possible.

Closes #35953

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35953 from drfloob:fix/35943 ff0b1db553
PiperOrigin-RevId: 608729800

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -2894,7 +2894,6 @@ static void next_bdp_ping_timer_expired_locked(
     grpc_core::RefCountedPtr<grpc_chttp2_transport> t,
     GRPC_UNUSED grpc_error_handle error) {
   GPR_DEBUG_ASSERT(error.ok());
-  GPR_ASSERT(t->next_bdp_ping_timer_handle != TaskHandle::kInvalid);
   t->next_bdp_ping_timer_handle = TaskHandle::kInvalid;
   if (t->flow_control.bdp_estimator()->accumulator() == 0) {
     // Block the bdp ping till we receive more data.