Remove SciHub references from code and docs

[j08lue]

The default endpoint for this package, SciHub, is dead, but there might be other DHuS instances out there for which it can still be useful.

To clarify what this package does and what not, we should remove all references to the original SciHub from the code and docs - at least where it is not a bigger effort.

Acceptance criteria

• Removed references to SciHub from docs and code