internal/frontend: delete redundant TestVulnsForPackage testing
internal/frontend: delete redundant TestVulnsForPackage testing

This is already tested in internal/vulns.TestVulnsForPackage.

Change-Id: I15f2248d0e37d614844efe494bc94a75d5531128
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/444678
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite