Delete backups/tmp after restore

What does this MR do?

• Deletes tmp/backups/tmp dir after restore.
• Fixes #22249 (closed)

How to setup and validate locally (strongly suggested)

1. Run

   bundle exec rake gitlab:backup:restore

2. After restore you'll see tmp dir still present inside tmp/backups/

3. After the fix:

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team