Strip dashes from ISBNs

[osma]

Currently our schema:issn values use the full form with dashes as it was written into MARC records, e.g. 951-0-15092-4. But this makes linking difficult since many other data sources use the form without dashes, and it's non-trivial to add the dashes.

The dashes should be stripped so that the value is only the number, e.g. 9510150924. Also the examples for schema:issn use this form.

[osma]

Note that the same doesn't apply to ISSNs, because they are conventionally written with the dash (XXXX-YYYY) always at the same position.