update flash script per pull request review

keyboards/dz60/keymaps/billiams_layout2/build_flash.sh

@@ -3,7 +3,4 @@
 # dfu-programmer atmega32u4 reset
 
 # run this in the qmk_firmware directory
-make dz60:billiams_layout2
-dfu-programmer atmega32u4 erase --force && \
-dfu-programmer atmega32u4 flash dz60_billiams_layout2.hex && \
-dfu-programmer atmega32u4 reset
+make dz60:billiams_layout2:flash

keyboards/dz60/keymaps/billiams_layout4/build_flash.sh

@@ -3,7 +3,4 @@
 # dfu-programmer atmega32u4 reset
 
 # run this in the qmk_firmware directory
-make dz60:billiams_layout4
-dfu-programmer atmega32u4 erase --force && \
-dfu-programmer atmega32u4 flash dz60_billiams_layout4.hex && \
-dfu-programmer atmega32u4 reset
+make dz60:billiams_layout4:flash