googlecode import: Clean up googlecode origin's origin_visits

When the loader-svn's runs will be finally done on googlecode import (that is all bug fixed + passes done), we need to clean up, for each origin, the multiple visits referenced. Indeed, they all represent the same visit (around may 2016).

We should only keep one, the latest.

Migrated from T958 (view on Phabricator)

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