Set proper AVR part for USBasp avrdude flashing (#7552)

* Set proper AVR part for USBasp avrdude flashing

* Remove `PROGRAM_CMD` stuff from rules.mk as they should not be needed anymore

* Missed the Plaid readme

* Remove PROGRAM_CMD from Mysterium

keyboards/handwired/hnah40/readme.md

@@ -10,7 +10,11 @@ Hardware Availability: https://github.com/vuhopkep/PCB/tree/master/Hnah40-Atmega
 
 Make example for this keyboard (after setting up your build environment):
 
-    make handwired/hnah40:default:program
+    make handwired/hnah40:default
+
+Flashing example for this keyboard:
+
+    make handwired/hnah40:default:flash
 
 ## Bootloader 
 use usbasploader from hsgw.

keyboards/handwired/hnah40/rules.mk

@@ -11,15 +11,9 @@ MCU = atmega328p
 #   ATmega328P   USBasp
 BOOTLOADER = USBasp
 
-# Flash program via avrdude, but default command is not suitable.
-# You can use hnah40:default:program
-PROGRAM_CMD = avrdude -c usbasp -p m328p -U flash:w:$(BUILD_DIR)/$(TARGET).hex
-
-
 # disable debug code
 OPT_DEFS = -DDEBUG_LEVEL=0
 
-
 # Build Options
 #   change yes to no to disable
 #