remove unused RedirectBackError class

[y-yagi]

Follow up to 333bfd8

[rails-bot]

r? @arthurnn

(@rails-bot has picked a reviewer for you, use r? to override)

[arthurnn]

thanks

[matthewd]

@arthurnn we should probably deprecate this

[arthurnn]

@matthewd as far as I understood, it was deprecated before 333bfd8 , and now it is just removed.

[matthewd]

My worry is that someone who was just rescuing the exception wouldn't've previously seen the deprecation. But as there isn't much more information for us to give them (e.g., a new name for the constant), on second thought this is probably fine.