Remove deprecations

Created by: pirj

This PR consists of a number of individual unrelated commits, better reviewed separately.

I don't mind sending them all as separate PRs. However, the changes are quite small and self-contained.

Depends on:

• https://github.com/rspec/rspec-support/pull/496
• https://github.com/rspec/rspec-expectations/pull/1290
• https://github.com/rspec/rspec-mocks/pull/1412
• https://github.com/rspec/rspec-rails/pull/2458

Not Done

There are a few things I'm afraid/reluctant of touching, specifically:

• described_class behaviour (see https://github.com/rspec/rspec-core/issues/1610#issuecomment-46316040, https://github.com/rubocop-hq/rspec-style-guide/issues/104)
• Rake task pattern/FilePath (https://github.com/rspec/rspec-core/blob/11928e0d0fc3265899c74a035c7ec48ce59ffed3/lib/rspec/core/rake_task.rb#L128) - seems that we'll remove a feature with no replacement