Remove deprecated find_or_create_namespace method from import/base_controller

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The new import service (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22809) extracts the common functionality so it can be shared between controllers and APIs.

The majority of import controllers (gitlab, bitbucket, bitbucket_server) still use the find_or_create_namespace method in app/controllers/import/base_controller.rb. This method will eventually be this will be replaced by app/services/import/base_service.rb as the importers are updated.