Content Relations should not be stored into db if no relations are request when a new site is created

[widoz]

Version Information

• WordPress: 4.6.9
• MultilingualPress: latest from master

Steps to Reproduce

1. Create two sites and related each together
2. Create a third site without relations and set the "Based On" to site 1 or 2 (doesn't matter)
3. Check the content relations are not created

What I Expected

No content relations are created because I haven't asked to correlate the site to another one.

What Happened Instead

I see the content relations in db where the site 3 is related with site 1