[internal] fix release preparation scripts

[pichlermarc]

While doing the most recent releases I discovered that the _lerna:remove_api and _lerna:remove_stable npm scripts do not work anymore as the packages are now linked with npm workspace. This causes every run of prepare_release:sdk:minor to also bump the API version, something that is not intended to happen.

A manual workaround was to remove the API package from the top-level package.json and re-add it later, but this process can be error prone. This issue is here to track fixing this script so that releases can be prepared with the available scripts again.