Revert a commit to fix sideloading of hasMany relationships, fixes #1773

Revert "hasMany relationship property are essentially readOnly, lets mark them as such."

This reverts commit a1501e5a3894295e36854805b44889f80cd46447.
This commit made hasMany relationship properties readonly, but since the RESTAdapter tries to set those properties directly when sideloading hasMany relationships from a JSON request, that broke that particular feature