internal/vulncheck/internal/buildinfo: remove unused field

This addresses failures on builders.

Change-Id: I065336fb6147450bf0da9740236bd6258f434528
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/507275
Run-TryBot: Zvonimir Pavlinovic <[email protected]>
Reviewed-by: Ian Cottrell <[email protected]>
Auto-Submit: Zvonimir Pavlinovic <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

internal/vulncheck: cleanup FetchVulnerabilities test

Change-Id: I3a8654ef9e12b7e761cb3f3b5c7aacd4e0a55eae
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/486807
Run-TryBot: Tatiana Bradley <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Tatiana Bradley <[email protected]>