Treat missing Atlas database user as successful delete

[Hashim1999164]

Summary

When destroy calls Delete for mongodbatlas_database_user and Atlas returns 404 USER_NOT_FOUND, treat that as success so apply can finish. This matches Read behavior and sibling resources such as teamprojectassignment.

Closes #4656

Test plan

1. Unit tests for the databaseuser package passed locally
2. Reviewers can confirm Delete ignores StatusNotFound the same way as teamprojectassignment

[marcabreracast]

Hi @Hashim1999164,
Thank you for reaching out with this PR! As my colleague @AgustinBettati mentioned in the original issue, we're currently discussing a provider-wide convention approach as this inconsistency behaviour affects several resources.

I f we go with your proposed approach, following our Contributing Guidelines in regards to Merging a Pull Request, we will create a branch from this PR and cherry pick your contribution.

I will keep this PR open for now so I can inform you when there's any relevant updates regarding this topic. Thank you for your patience!

[Hashim1999164]

@marcabreracast yeah that makes sense if a bunch of resources do the same thing. no rush, cherry pick is fine whenever you land on the convention.