List<Contact> cont = new List<Contact>();
Map<String, inscor__Customer_Quote__c> cqMap = new Map<String, inscor__Customer_Quote__c>();

for (inscor__Customer_Quote__c cq : [SELECT Id, inscor__Contact__c FROM inscor__Customer_Quote__c]) {
    cqMap.put(cq.inscor__Contact__c, cq);
}

for (Contact c : [SELECT Id FROM Contact WHERE inscor__External_Reference_ID__c = null]) {
    if (!cqMap.containsKey(c.Id)) {
        cont.add(c);
    }
}

delete cont;