Remove threat mappings accidentally brought in with indicator work

I copied the entirety of the `threat` mappings in order to get the
`threat.indicator` ones, but it looks like these were added at some
point too.

I'd rather these not be added incidentally. If we need them, we should
do so explicitly.

x-pack/plugins/security_solution/server/lib/detection_engine/routes/index/ecs_mapping.json

@@ -2630,28 +2630,6 @@
               "reference": {
                 "ignore_above": 1024,
                 "type": "keyword"
-              },
-              "subtechnique": {
-                "properties": {
-                  "id": {
-                    "ignore_above": 1024,
-                    "type": "keyword"
-                  },
-                  "name": {
-                    "fields": {
-                      "text": {
-                        "norms": false,
-                        "type": "text"
-                      }
-                    },
-                    "ignore_above": 1024,
-                    "type": "keyword"
-                  },
-                  "reference": {
-                    "ignore_above": 1024,
-                    "type": "keyword"
-                  }
-                }
               }
             }
           }