[teamcity/gitian] Do not remove src tarball from results
[teamcity/gitian] Do not remove src tarball from results

Summary:
Reasons to delete the src tarball:
* Save disk space
Reasons to keep it:
* Easily package a release from TeamCity (if needed)
* Verify and/or manually test the src tarball.
* TeamCity automatically cleans up old build artifacts, so disk space is not really an issue.

I'm in favor of keeping it.

Test Plan: TeamCity gitian build (see comments)

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3864

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