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/feather-nrf52840.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["nrf52840", "nrf52840-s140v6-uf2"],
     "build-tags": ["feather_nrf52840"],
-    "serial-port": ["acm:239a:8029", "acm:239a:802a"],
+    "serial-port": ["239a:8029", "239a:802a"],
     "msd-volume-name": "FTHR840BOOT"
 }