Remove `image-function` but reserve its ID for future use

[jamesnw]

image() has no implementations, but was tracked in BCD. mdn/browser-compat-data#25190 removed it from BCD.

Unless and until image() has a real prospect of implementation (e.g., an intent, a bug with associated commits, etc.), then we shouldn't publish this feature. We ought to:

• withdraw it in the next major release
• move the feature description to drafts
• somehow reserve the feature ID (e.g., make sure there's an error if someone tries to make a non-draft that shadows the draft ID)

[jamesnw]

@ddbeck FYI- this mirrors #2339 and will be another item to address with the next BCD version.