Remove Rugged and shell code from Gitlab::Git

What does this MR do?

What are the relevant issue numbers?

Closes gitaly#1124 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides