spec_helper: remove cruft

[ndushay]

Why was this change made? 🤔

• remove unused code
• It's annoying to have to look in spec_helper for definitions, especially if they are only used in one spec file.
• rename rspec matcher to be more accurate

How was this change tested? 🤨

⚡ ⚠ If this change has cross service impact, run integration tests that do IMAGE ACCESSIONING (e.g. create_preassembly_image_spec) and/or test in [stage|qa] environment, in addition to specs. ⚡

[jcoyne]

Feel free to merge after fixing/ignoring rubocop errors.