Clean the checks for old artifacts from buildpack-verification pipeline

[ivanovac]

The verification task is checking ALL git tags in the repository's history, which includes the old commits for binaries like 3.0.0-preview7
from 2019 that were hosted on the old S3 bucket.

• dotnet-aspnetcore: 3.0.0-preview7 (ancient preview from 2019)
• dotnet-runtime: 3.0.0-preview7
• dotnet-sdk: 3.0.100-preview7
• URLs point to s3.amazonaws.com/pivotal-buildpacks

Clean the checks for the obsolete artifacts

[ramonskie]

ref: cloudfoundry/public-buildpacks-ci-robots#12