x/pkgsite: remove stdlib branches

[jba]

pkgsite has custom code for fetching two stdlib branches, dev.boringcrypto and dev.fuzz. The first hasn't had a commit for a year and the second for over 2 years. Neither one is in the DB now. I'm going to remove the code for these.

[gopherbot]

Change https://go.dev/cl/557415 mentions this issue: internal/stdlib: remove support for special branches