scripts

* Remove redundant `pre-release.sh` script

This is now a no-op (there are no `matrix_lib` fields in package.json), so we
may as well remove it.

* Remove redundant `post-merge-master` script

Just as pre-release is a no-op, so is this

* Remove redundant switch_package_to_release script

Once more: this script is a no-op.