Remove minimum and maximum of _extruder_nr settings

This way the validation won't get an error any more. There is no validation. It's an enum anyway.

Contributes to issue CURA-1716.

resources/definitions/fdmprinter.def.json

@@ -2724,9 +2724,7 @@
                     "label": "Platform Adhesion Extruder",
                     "description": "The extruder train to use for printing the skirt/brim/raft. This is used in multi-extrusion.",
                     "type": "extruder",
-                    "default_value": 0,
-                    "minimum_value": "0",
-                    "maximum_value": "machine_extruder_count - 1",
+                    "default_value": "0",
                     "settable_per_mesh": false,
                     "settable_per_extruder": false
                 },
@@ -2735,9 +2733,7 @@
                     "label": "Support Extruder",
                     "description": "The extruder train to use for printing the support. This is used in multi-extrusion.",
                     "type": "extruder",
-                    "default_value": 0,
-                    "minimum_value": "0",
-                    "maximum_value": "machine_extruder_count - 1",
+                    "default_value": "0",
                     "enabled": "support_enable",
                     "settable_per_mesh": false,
                     "settable_per_extruder": false,
@@ -2747,10 +2743,8 @@
                             "label": "Support Infill Extruder",
                             "description": "The extruder train to use for printing the infill of the support. This is used in multi-extrusion.",
                             "type": "extruder",
-                            "default_value": 0,
+                            "default_value": "0",
                             "value": "support_extruder_nr",
-                            "minimum_value": "0",
-                            "maximum_value": "machine_extruder_count - 1",
                             "enabled": "support_enable",
                             "settable_per_mesh": false,
                             "settable_per_extruder": false
@@ -2760,10 +2754,8 @@
                             "label": "First Layer Support Extruder",
                             "description": "The extruder train to use for printing the first layer of support infill. This is used in multi-extrusion.",
                             "type": "extruder",
-                            "default_value": 0,
+                            "default_value": "0",
                             "value": "support_extruder_nr",
-                            "minimum_value": "0",
-                            "maximum_value": "machine_extruder_count - 1",
                             "enabled": "support_enable",
                             "settable_per_mesh": false,
                             "settable_per_extruder": false
@@ -2773,10 +2765,8 @@
                             "label": "Support Roof Extruder",
                             "description": "The extruder train to use for printing the roof of the support. This is used in multi-extrusion.",
                             "type": "extruder",
-                            "default_value": 0,
+                            "default_value": "0",
                             "value": "support_extruder_nr",
-                            "minimum_value": "0",
-                            "maximum_value": "machine_extruder_count - 1",
                             "enabled": "support_enable",
                             "settable_per_mesh": false,
                             "settable_per_extruder": false