removed unused gmm-config fields

active-crust/gmm-config.json

 {
-  "max-distance": 300.0,
-  "epistemic-model": "NGA_WEST2",
-  "epistemic-tree": [
-    { "id": "+uₑ", "weight": 0.185, "value": 1.0 },
-    { "id": "~uₑ", "weight": 0.63, "value": 0.0 },
-    { "id": "-uₑ", "weight": 0.185, "value": -1.0 }
-  ]
+  "max-distance": 300.0
 }

stable-crust/gmm-config.json

 {
-  "max-distance": 1000.0,
-  "epistemic-model": null,
-  "epistemic-tree": null
+  "max-distance": 1000.0
 }

subduction/interface/gmm-config.json

 {
-  "max-distance": 1000.0,
-  "epistemic-model": null,
-  "epistemic-tree": null
+  "max-distance": 1000.0
 }

subduction/slab/gmm-config.json

 {
-  "max-distance": 300.0,
-  "epistemic-model": null,
-  "epistemic-tree": null
+  "max-distance": 300.0
 }