Remove default pull request template (#1250)

### Rationale

Removes this repository's default pull request template
(`pull_request_template.md`).

### Related Pull Requests

- LabKey/server, LabKey/nbconvert-service, LabKey/distributions,
LabKey/targetedms — same change on branch `fb_remove_pr_template`.

### Changes

- Delete `pull_request_template.md`.

Author: labkey-klum