S3 backend: include path prefix in removal requests.

[mbechler]

DELETE requests, both for temporary files and no longer referenced
packages, lacked the configured path prefix and therefor were not
removed if a prefix is configured.

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Adds the path prefix to Remove() so that temporary and unreferenced pool files are properly removed.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[smira]

nice catch!