Remove overrides that didn't have any effect

This works due to our new test that tests for that case. A common failure case.

resources/definitions/prusa_i3_mk3.def.json

@@ -24,17 +24,14 @@
         "machine_depth": { "default_value": 210 },
         "machine_center_is_zero": { "default_value": false },
         "material_diameter": { "default_value": 1.75 },
-        "material_bed_temperature": { "default_value": 60 },
         "machine_nozzle_size": { "default_value": 0.4 },
         "layer_height": { "default_value": 0.15 },
         "layer_height_0": { "default_value": 0.2 },
         "retraction_amount": { "default_value": 0.8 },
         "retraction_speed": { "default_value": 35 },
-        "retraction_retract_speed": { "default_value": 35 },
-        "retraction_prime_speed": { "default_value": 35 },
         "adhesion_type": { "default_value": "skirt" },
         "machine_head_with_fans_polygon": { "default_value": [[-31,31],[34,31],[34,-40],[-31,-40]] },
-        "gantry_height": { "default_value": 28 },
+        "gantry_height": { "value": 28 },
         "machine_max_feedrate_z": { "default_value": 12 },
         "machine_max_feedrate_e": { "default_value": 120 },
         "machine_max_acceleration_z": { "default_value": 500 },