fix(schema): avoid propagating toObject.transform and toJSON.transform option to implicitly created schemas

[vkarpov15]

Fix #13599

Summary

#13599 pointed out that we introduced a potentially breaking change in #13396: transform() option propagates to implicitly created schemas, which means existing transform functions need to account that they'll run on subdocuments. This PR undoes that backwards breaking change by explicitly omitting transform from the toJSON and toObject options propagated down to implicitly created schemas.

Examples

[hasezoey]

should this be reverted again for 8.0 as a breaking change?

[vkarpov15]

I don't think we should revert this PR for 8.x. I think avoiding propagating transform down to implicitly created schemas is better UI design. Many transform functions are only relevant to the current schema, and not relevant to child schemas, so propagating transform causes more friction. The major benefit of propagating toObject and toJSON is propagating virtuals and getters.