Remove obsolete :os_image_version argument from stemcell:build

Originally introduced in change https://github.com/cloudfoundry/bosh/commit/02d7e67cd7de4aa07ecd8c374a84cbda982c29db,
and reverted in change https://github.com/cloudfoundry/bosh/commit/f6f9c8fa7fe960c0551e1dc58b6387e8ac719d85,
it came back in change https://github.com/cloudfoundry/bosh/commit/2754b45fe0ecd56beedbd75732cec883c754af54

Seems like this is obsolete (at least for now), so let's remove it.