removed duplicate page.

this file isn't being linked to from anywhere within the site.

```
stefan@pittiplatsch: ~/dev/projects/ucsf-history-website on master
$ grep -rl '1868_goldrush-05202008.html'
.git/index

```

looks like a backup file that was (accidentally) added to version control back-when.
compare:

original:
http://history.library.ucsf.edu/1868_goldrush.html

vs
copy (with some minor layout borkage)

http://history.library.ucsf.edu/1868_goldrush-05202008.html

in sum: kruft, can be removed.