Replace config.json's Model/Plant blocks with one shared default in brewpi.py

Both blocks were nearly identical (config.json.sim's differing M and
unused gain key being the only divergence), so there's no need for
per-config tuning here. DEFAULT_PLANT_PARAMS in brewpi.py now feeds
both the real Pot plant and the temperature controller's
Smith-predictor model. No demo scripts read these config keys, so
nothing else needed updating.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgR9tPaSzFkAwRAUyeaaCD

config.json.templ

@@ -32,18 +32,6 @@
 			"HeatIdle": 1.0
 		}
 	},
-	"Model": {
-		"C": 4190,
-		"M": 25,
-		"L": 0.05,
-		"Td": 15
-	},
-	"Plant": {
-		"C": 4190,
-		"M": 25,
-		"L": 0.05,
-		"Td": 15
-	},
 	"Heater": {
 		"port": "/dev/ttyUSB0",
 		"speed": "115200"