Remove default value from `organization_id` field for snippets

What does this MR do and why?

Contributes to #460828 (closed)

Based on the guidance from https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#changing-column-defaults.

Problem

After !157768 (merged), organization_id should be set by the application. Default value in database is not necessary anymore.

Solution

Remove default value from organization_id