Unname layers

[teunbrand]

Hi Daniel (or other maintainers)!

We've done a revdepcheck for the upcoming ggplot2 release and found that ggsurvfit gets stuck on layers having names.
The .extract_arguments_from_attr() function fails to correctly format its result whever x$layers is a named list.

What changes are proposed in this pull request?

A change to make .extract_arguments_from_attr() more stable by unnaming the layers.

If there is an GitHub issue associated with this pull request, please provide link.

tidyverse/ggplot2#6287

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When the branch is ready to be merged into master:

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[ddsjoberg]

Thank you @teunbrand ! Do you have a target date for the next ggplot2 release?

[teunbrand]

The release date hasn't been discussed yet, but I'll let you know once I got more clarity on this!

[teunbrand]

We discussed the release and we think before summer, so May-ish if we have to give a ballpark.

[ddsjoberg]

awesome, thank you @teunbrand !