Update/remove links to deleted getty.edu pages

[geealbers]

Per Betsy Brand in the museum, two getty.edu Museum pages have been deleted, and so have created dead links where we references them in the book. (cc @ruthlane)

Page: https://www.getty.edu/publications/artistryinbronze/introduction/ Link to remove: Footnote 1, http://www.getty.edu/​museum/​symposia/​pdfs_bronze/​bronzecongress_getty_schedule.pdf

Page: https://www.getty.edu/publications/artistryinbronze/large-scale-bronzes/8-mattusch/index.html Link to remove: Bibliography Melillio, https://www.getty.edu/museum/symposia/restoring_bronzes/index.html

[geealbers]

One note: the second removed link leads to a 404 page that includes a link to an version of the page that's archived on the Internet Archive. So perhaps we can leave it as is in the book? The first link, however, is a PDF and doesn't lead to an archived version.