Support PV images from polyline region

[pford]

Description Draft PR until frontend development allows testing.

• What is implemented or fixed? Mention the linked issue(s), if any. Implements #1341
• How does this PR solve the issue? Give a brief summary. Removes check for polyline region from PV image generation. Will continue to fail for annotation polyline.
• Are there any companion PRs (frontend, protobuf)? CARTAvis/carta-frontend#2302
• Is there anything else that testers should know (e.g. exactly how to reproduce the issue)? (Pending frontend implementation) Create a polyline and request PV image using this region.

Checklist

• [x] changelog updated / ~no changelog update needed~
• [x] e2e test passing / corresponding fix added / new e2e test created
• [ ] ICD test passing / corresponding fix added / new ICD test created
• [x] ~protobuf updated to the latest dev commit~ / no protobuf update needed
• [x] ~protobuf version bumped~ / protobuf version not bumped
• [x] added reviewers and assignee
• [ ] added ZenHub estimate, milestone, and release

[kswang1029]

@pford I tested with the frontend PR https://github.com/CARTAvis/carta-frontend/pull/2385 and found that when we select a polyline region as the pv cut, the resulting PV image is with the "offset" axis instead of the "distance" axis (like what we have for the spatial profile).

[github-actions[bot]]

Package	Line Rate	Health
src.Cache	72%	➖
src.DataStream	44%	➖
src.FileList	67%	➖
src.Frame	36%	❌
src.HttpServer	42%	➖
src.ImageData	28%	❌
src.ImageFitter	83%	✔
src.ImageGenerators	44%	➖
src.ImageStats	75%	✔
src.Logger	37%	❌
src.Main	52%	➖
src.Region	69%	➖
src.Session	4%	❌
src.Table	52%	➖
src.ThreadingManager	67%	➖
src.Timer	85%	✔
src.Util	40%	➖
Summary	46% (8631 / 18816)	➖