Allow minimum layer time to be higher than the fan speed min/max threshold

Previously that wasn't possible because the division that calculated the slope of the fan speed ramping up towards the minimum layer time couldn't deal with negative values. Now it deals with that properly and in the limit the regular fan speed will be used.

Fixes #2328 and CURA-4272.

resources/definitions/fdmprinter.def.json

@@ -3379,7 +3379,6 @@
                     "unit": "s",
                     "type": "float",
                     "default_value": 10,
-                    "minimum_value": "cool_min_layer_time",
                     "maximum_value_warning": "600",
                     "settable_per_mesh": false,
                     "settable_per_extruder": true