php: Drop alternative Stretch images

Fix Drone too.

.drone.yml

@@ -145,34 +145,6 @@ steps:
       - stretch
       - '7.0'
 
-- name: build php (Stretch PHP 7.2)
-  image: plugins/docker
-  depends_on: []
-  settings:
-    username:
-      from_secret: docker_username
-    password:
-      from_secret: docker_password
-    context: ./php
-    dockerfile: php/Dockerfile.stretch72
-    repo: evolix/php
-    tags:
-      - stretch72
-      - '7.2'
-
-- name: build php (Stretch PHP 7.3)
-  image: plugins/docker
-  depends_on: []
-  settings:
-    username:
-      from_secret: docker_username
-    password:
-      from_secret: docker_password
-    context: ./php
-    dockerfile: php/Dockerfile.stretch73
-    repo: evolix/php
-    tags: stretch73
-
 - name: build php (Buster)
   image: plugins/docker
   depends_on: []