fix: Lechapt-Calmon JSON calculator configuration: remove unused defaultMaterial field

refs #589

src/app/calculators/lechaptcalmon/config.json

@@ -2,7 +2,6 @@
     {
         "id": "fs_materiau",
         "type": "fieldset",
-        "defaultMaterial": "UnlinedCastIronCoarseConcrete",
         "fields": [
             {
                 "id": "select_material",
@@ -31,4 +30,4 @@
         "selectIds": [ "select_material" ],
         "help": "hyd_en_charge/lechapt-calmon.html"
     }
-]
\ No newline at end of file
+]