extruder: Consistently use "extruder" for the primary extruder

No longer allow the primary extruder to be named "extruder0".  This
avoids internal and external confusion between the printer objects and
config section names.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/generic-duet2-duex.cfg

@@ -182,10 +182,10 @@ run_current: 1.000
 sense_resistor: 0.051
 
 #On drive E0
-[extruder0]
+[extruder]
 step_pin: PD5
 dir_pin: PA1
-enable_pin: !PC6, tmc2660_extruder0:virtual_enable
+enable_pin: !PC6, tmc2660_extruder:virtual_enable
 step_distance: .002
 nozzle_diameter: 0.400
 filament_diameter: 1.750
@@ -199,7 +199,7 @@ pid_Kd: 114
 min_temp: 0
 max_temp: 250
 
-[tmc2660 extruder0]
+[tmc2660 extruder]
 cs_pin: PC17
 spi_bus: usart1
 microsteps: 16
@@ -297,10 +297,10 @@ max_temp: 130
 [fan]
 pin: PC23
 
-# Fan1 controlled by extruder0
+# Fan1 controlled by extruder
 [heater_fan nozzle_cooling_fan]
 pin: PC26
-heater: extruder0
+heater: extruder
 heater_temp: 45
 fan_speed: 1.0