[firebase_storage] Remove StorageTaskEvent prints

[creativecreatorormaybenot]

Reason

It is generally pretty annoying to see prints in your logs from sources you do not control and in this case, any user could manually add print statements (or even better, other kinds of notifications) very easily.
More importantly, this actually becomes a problem when running multiple uploads simultaneously. Here is a list of issues I encountered:

1. There is no indication as to which upload the print statement is referring to, it is just a continuous stream of EVENT StorageTaskEventType.progress and EVENT StorageTaskEventType.success.

2. There is no way to disable these prints if I do not want them to clutter my logs.

3. This does impact performance and that is a problem when considering the previous point.

4. They seem to be delayed? I am not entirely sure about this, but I added an element to my UI that waited for Future.wait(futures), where futures was a list of onComplete's and that element showed that all uploads did complete long before the logging stopped.

Implementation

I decided to essentially do nothing for the implementation (only remove the print call) and here is why:

• There already is a way to manually print the upload status. I simply added explanation of how to do this both to README.md and CHANGELOG.md.

• I would need to add an implementation to the plugin to make this behavior optional out of the box, which seems redundant to me as there already is a stream that is meant to handle cases like this.

Conclusion

I feel like there is no good answer to the question "Why do we need to print the upload status out of the box in the first place?".
If the developer wants to know about their upload status, they should listen to the stream and this even encourages to find different, possibly better ways, to notify about the upload status than simply printing to the logs.

[collinjackson]

Thanks. LGTM