Remove deprecated rebase code after removal of the two_step_rebase feature flag

What does this MR do?

MR !23546 (merged) removed the two_step_rebase feature flag (as described in #29077 (closed) ), which left the functions Repository#rebase_deprecated and (as a result of that) Gitlab::Git::Repository#rebase_deprecated unreachable.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #29077 (closed)