Remove the source manifest; tags come from repository topics

The build generates the full manifest, and when the source manifest has no tags it collects them from the repository topics (Build-PSModuleManifest reads repositoryTopics). Repo topics set: confluence, atlassian, psmodule, powershell, powershell-module, pwsh, rest, rest-api. A missing source manifest is fully supported (the build starts from an empty manifest and derives version, description, project/license/icon URIs, and exports).

Author: MariusStorhaug