Cleanup SafelyChangeColumnDefault for snippets

!160048 (merged) introduced a migration to update a default value for snippets table.

This process has two steps:

1. Add the SafelyChangeColumnDefault concern to the model and change the default in a post-migration (Done in !160048 (merged))
2. Clean up the SafelyChangeColumnDefault concern in the next minor release - should be done in scope of this issue.

To finalize the migration, we should cleanup SafelyChangeColumnDefault from the code after %17.3.