`elements_miniscript::confidential::slip77` duplicates code from `elements::slip77`

[LeoComandini]

Those have small differences but they are basically the same thing,
which one should be used by downstream libraries?

[apoelstra]

confidential::slip77 should be used downstream. We can delete the top-level module. Maybe we should re-export it though so that it has a more obvious location.

[RCasatta]

It should be closed since it has been removed from rust-elements

[apoelstra]

Closing. Though it may make sense to open an issue to move slip77 specifically back. Unsure. My feeling is that we want to discourage slip77 going forward in favor of "CT descriptors".