Checkmk Werk 15508: time_period: verify time_period usages before deletion

ID: 15508 Title: time_period: verify time_period usages before deletion Component: REST API Level: 1 Class: Bug fix Version: 2.3.0b1 Prior to this werk, the REST API did not check whether a time period to be deleted was referenced by other objects. This caused a conflict after deletion. This werk fixes this by preventing the REST API from performing a deletion if the time period is currently being referenced.