Unable to remove Seasonality

I have sales data which is seasonal and has no trend. The frequency of this series is 15 mins. I don't know how to compute the exact period of seasonality - whether it is daily or weekly or monthly or yearly. But, from plotting it, I think there is a yearly pattern.

I tried removing seasonality before forecasting by lagging the series by a year and differencing the two but even the result has a yearly pattern.

Code with what I've tried can be found in - this notebook

My questions -

1. Should I remove seasonality to forecast sales for the next month?
2. The method I have described above does not work. What can I do alternatively?
3. Can I use methods like decision trees to forecast sales for the next month? Should I derive features from the date alone to form the training set and use sales as label?