Remove istanbul@0.4.5 JavaScript package

istanbul@0.4.5 has been deprecated at the upstream. On gitlab-ce, it was introduced in !5052 (merged) (and modified with version management with npm in !7548 (merged)), but it seems unused now. Thus "istanbul": "^0.4.5" in devDependencies of package.json can be removed.

yarn install is outputting as follows (omitted):

warning istanbul@0.4.5: This module is no longer maintained, try this instead:
  npm i nyc
Visit https://istanbul.js.org/integrations for other alternatives.