remove deprecated and unused Upload class

[jtnord]

Remove the Upload class

Code was deprecated > 3 years ago in #208 and is unused.

usage-in-plugins showed no usage f this in either Jenkinsci or cloudbees' plugins.

Testing done

mvn hpi:run
uploaded the test cert provided with the repo.
created the following pipeline:

node {
  withCredentials([certificate(aliasVariable: 'ALIAS', credentialsId: 'ffad04d8-48f8-4f34-8726-82fb1a746fd3', keystoreVariable: 'KS', passwordVariable: 'PW')]) {
    bat 'keytool -list -v -keystore %KS% -storepass %PW%'
  }
}

checked the pipeline ran and output was expected.

updated the description of the credential and saved.
ran the job again and validated the output is still expected

Submitter checklist

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• Ensure you have provided tests - that demonstrates feature works or fixes the issue