Remove rstan3 directory

[jeffreypullin]

Summary:

Remove the (obsolete?) rstan3 directory following up on #898

Intended Effect:

Simplification of the structure of the repository

How to Verify:

These changes should be non-functional.

Reviewer Suggestions:

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Jeffrey Pullin

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: GPLv3 (http://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[hsbadr]

@jeffreypullin Agreed. I've picked this in #887 at 23a7869.

[jeffreypullin]

@hsbadr I'm happy for this to be picked into #887 but given how big #887 already is it might just be easier for @bgoodri to review and merge separately.

[hsbadr]

@hsbadr I'm happy for this to be picked into #887 but given how big #887 already is it might just be easier for @bgoodri to review and merge separately.

Yes. After this gets merged, it'll have to be applied to #887 anyway to resolve merging conflicts. Hope both will get merged & we start from there.