Remove snyk-scan-pr.yml (#1631)

This workflow is causing errors on each PR:

  Snyk is missing auth token in order to run inside CI. You must include your API token as an environment value: `SNYK_TOKEN=12345678`
  Error: Process completed with exit code 2.

As discussed on #1608, the preferred workflow will run weekly, not on PR.

Signed-off-by: Cary Phillips <cary@ilm.com>