Template dashboards that are missing in the template import file are not deleted when importing template with delete option set

Problem description: If a user imports an already existing template (to update it for example) with "Delete missing" set to true for "Template dashboards " and the import file doesn't have dashboards that exists in the current template version in frontend, then these dashboards will not be deleted after import.

Steps to reproduce:

1. Export any existing template that has dashboards and graphs
2. In the exported file delete all dashboards and all graphs
3. Import this modified file with "Delete missing" flag set to true for Graphs and Templated dashboards

Result: Graphs are deleted after import, but dashboards are still there.
Expected: Dashboards that are missing in the import file should be deleted if the "Delete missing" flag is set