remove .gitattributes (#740)

Contributes to rapidsai/build-planning#31

Removes `.gitattributes` files.

These have been here since the beginning of the project (048d53e), 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` (#615). `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)
  - Gregory Lee (https://github.com/grlee77)

URL: #740