Add ClearProvider again to ActivityRegistry

[truthz03]

Hi,

for my requirements I need to remove a custom activity provider (add/remove plugins).
WIthout removing it from the _providedActivityDescriptors it is not possible to delete my plugin files because it holds a reference.

With version 3.1.3 there was a ClearProvider method inside the ActivityRegistry class.

Why was this removed?
Is it possible to add it again?

[sfmskywalker]

I don't know why it was removed, but we should restore it. Please feel free to open a PR, but it's OK if you don't - I'll add this issue to the 3.4 milestone anyway.