Bad automatic change of main WP when we edit a route

[fbunoz]

Describe the bug
When we edit a route whose main WP is undefined, one of linked WP is set as main WP.
As it is allow to keep the main WP undefined (useful when it isn't necessary to put a WP name at the start of the title), if we don't be careful, we save this change.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://www.camptocamp.org/routes/49711/fr/tour-des-aiguilles-de-l-argentiere-col-de-la-croix-col-de-la-combe-madame-breche-du-pin
2. Click on edit button
3. Open Information section
4. See that main WP is set with "Col de la Combe Madame"

Expected behavior
The good behaviour is to not change the main WP selection when we edit a route.
But when we create a route, it's useful to set automatically the main WP.

This bug exist since this enhancement has been developped : #759