targets: remove "acm:"` prefix for USB vid/pid pair

This prefix isn't actually used and only adds noise, so remove it.

It may have been useful on Linux that makes a distinction between
/dev/ttyACM* and /dev/ttyUSB* but it isn't now. Also, it's unlikely that
the same vid/pid pair will be shared between an acm and usb driver
anyway.

targets/pybadge.json

@@ -4,7 +4,7 @@
     "serial": "usb",
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
-    "serial-port": ["acm:239a:8033"],
+    "serial-port": ["239a:8033"],
     "msd-volume-name": "PYBADGEBOOT",
     "msd-firmware-name": "arcade.uf2"
 }