mergify: drop parameter `rebase_fallback`

[mtitov]

Mergify will always report errors in the future if a rebase merge is impossible

based on this comment: https://github.com/ExaWorks/SDK/pull/173#issuecomment-1494539359

[mtitov]

Is that because .mergify.yml is read from the base branch?

@andre-merzky yeah, I would think so (in that error message it still refers to the parameter, which I've removed)