Only execute system hooks once when pushing tags

What does this MR do?

At present, we execute system hooks twice when pushing a tag - once with information about the last commit in the tag, and the tag's message, and once without.

I don't know why we do this, it's very arguably a bug, and it means pushing branches and pushing tags differ from each other in a way that interferes with my Git::{Branch,Tag}PushService refactor scheduled for %11.10: https://gitlab.com/gitlab-org/gitlab-ce/issues/15463 . So, let's try removing the system hook duplicate that has less information in it.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #52560 (closed)