extensions: remove url from copied manifest

Created by: tjkandala

Closes #717 (closed).

• Update updatePropertyInManifest to remove property when value is an empty string.
• Remove url property from manifest before publishing to private registry.

This is a backwards-compatible change, so can it be part of a patch release? And is the changelog entry unnecessary in this case then?

Test plan

• Added a test case for removing properties from an extension manifest.
• Built with go install and tested src ext copy to confirm that the command still works and that url is no longer included in the manifest.