remove .gitattributes (#603)

Contributes to rapidsai/build-planning#31

Removes `.gitattributes` files.

These were added in #66 for use with `versioneer`. Per the `git` docs ([link](https://git-scm.com/docs/gitattributes#_export_subst)), setting the attribute `export-subst` on a file via a `.gitattributes` tell `git` to replace placeholders in the file with some `git` information.

This is no longer done in `_version.py` files in this project, and this project no longer uses `versioneer` (#497). `rapids-build-backend` handles storing git commit information in the published packages.

## Notes for Reviewers

For more details, see rapidsai/build-planning#31 (comment)

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Mike Sarahan (https://github.com/msarahan)
  - Ajay Thorve (https://github.com/AjayThorve)

URL: #603