git: Remove Repository#rebase_in_progress?

What does this MR do?

In [1] the implementation of UserRebaseConfirmable was rewritten in Go. This rewrite no longer uses worktrees. Because of this, Gitaly is pretty much stateless in regard of a rebase. Also in [2] some work was done to keep the status of a rebase completely in the model, and no longer ask Gitaly for the status.

This change removes all calls of IsRebaseInProgress and makes the code completely rely on internal status kept in the MergeRequest model.

1. gitaly!2876 (merged)
2. d31b733f (Only read rebase status from the model, 2019-08-15)

Issue: gitaly#3614 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.