Remove rimraf as (unused) dev dependency

rifraf is required by Bower. Having rifraf specified as our own dev
dependency stopped it from being installed correctly on npm install
when invoked with the `--production` flag and in the presence of an
`npm-shrinkwrap.json` (since 67d95139).

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2368 from opf/fix/packager-npm-bower-missing-dependencies

Remove (unused) rimraf as dev dependency