ci: remove broken Assign to One Project workflow (#216)

## Summary

The add-to-project.yml workflow runs the
srggrs/assign-one-project-github-action on every pull request and issue.
This action has been failing consistently, which marks every pull
request as unstable regardless of the actual test results.

This removes only that workflow file. The separate
auto-assign-project.yml workflow, which adds newly opened issues to the
GRIDAPPSD project board via actions/add-to-project, is unrelated and
untouched. It only fires on issue open events, never on pull requests,
and is not the source of the failing check.

## Test plan

- Confirmed via the GitHub API that the failing check named Assign to
One Project on recent pull requests corresponds to the
assign_one_project job defined in add-to-project.yml.
- Confirmed no other workflow references
srggrs/assign-one-project-github-action.
- After merge, verify a new pull request no longer shows the Assign to
One Project check and no longer reports an unstable mergeable state due
to it.

Author: craigpnnl