Add SSRN urls to CompositeIdFetcher

[trickypr]

Allows importing (e.g. via pasting) of both short and long SSRN URLs via their associated DOI.

This also removes some dead code that was not cleaned up in #12009.
Closes #11687

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[koppor]

Minor comments. All in all, looks good!

[koppor]

@trickypr Thank you for the work! Maybe, you can tackle another issue, too?