Remove the Heroku deploy-button manifest (#466)

app.json only served the "Deploy to Heroku" button, which #464 removes
from the readme. blurb-it moves to cabotage, which builds from the
Dockerfile and ignores this file.

It was the only JSON file in the repository, so drop the check-json
pre-commit hook too; otherwise check-hooks-apply fails.

Author: Mariatta

.pre-commit-config.yaml

@@ -10,7 +10,6 @@ repos:
       - id: check-added-large-files
       - id: check-case-conflict
       - id: check-merge-conflict
-      - id: check-json
       - id: check-yaml
       - id: debug-statements
       - id: end-of-file-fixer