Remove minimum warning value for z-hop height

It was giving a warning for 0.8mm polypropylene because that was set to 0.5mm hop. It was decided that a minimum value at all for z-hop is nonsense because you can just disable z-hop without getting a warning. I'm still setting it to zero because negative will still go wrong in your print.

Contributes to issue CURA-3960.

resources/definitions/fdmprinter.def.json

@@ -2726,7 +2726,7 @@
                     "unit": "mm",
                     "type": "float",
                     "default_value": 1,
-                    "minimum_value_warning": "0.75 * machine_nozzle_size",
+                    "minimum_value_warning": "0",
                     "maximum_value_warning": "10",
                     "enabled": "retraction_enable and retraction_hop_enabled",
                     "settable_per_mesh": false,