Remove unused exporter from openTelemetryProviderImpl

[k24dizzle]

Doesn't seem like this field is actually used while meter, config, and server are:

temporal/common/metrics/opentelemetry_provider.go

Lines 126 to 136 in cdd1a21

	func (r *openTelemetryProviderImpl) GetMeter() metric.Meter {
	return r.meter
	}
	func (r *openTelemetryProviderImpl) Stop(logger log.Logger) {
	ctx, closeCtx := context.WithTimeout(context.Background(), time.Second)
	defer closeCtx()
	if err := r.server.Shutdown(ctx); !(err == nil || err == http.ErrServerClosed) {
	logger.Error("Prometheus metrics server shutdown failure.", tag.Address(r.config.ListenAddress), tag.Error(err))
	}
	}

[CLAassistant]

All committers have signed the CLA.