Remove redundant waits from mr rebase button

What does this MR do and why?

Capybara will automatically wait until the element is 'enabled' before clicking it, so we should be able to remove these additional checks and simplify our code.

References:

• https://github.com/teamcapybara/capybara/blob/5521da18729c3337d63351e7c4a59faf13538837/lib/capybara/spec/session/click_button_spec.rb/#L477-L481
• https://github.com/teamcapybara/capybara/issues/771

How to set up and validate locally

Appears to be a single use of this in the suite, so can be tested out using bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/rebase_merge_request_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.