[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