Remove pa11y #96

pa11y and pa11y-ci aren't needed. They're only referenced by two package.json scripts (a11y, a11y:json) and two .gitignore entries, and they pull in Puppeteer and a bundled Chromium on every yarn install, including in CI. Dependabot has opened major-version updates for both (#93, #95).

• Remove pa11y and pa11y-ci from devDependencies and regenerate yarn.lock.
• Remove the a11y and a11y:json scripts and the /.pa11yci.json and /pa11y-report.json .gitignore entries.
• yarn install --frozen-lockfile and npm run production still succeed.
• Close Bump pa11y-ci from 4.0.1 to 5.0.0 #93 and Bump pa11y from 9.0.1 to 10.0.0 #95.