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/m5stack.json

@@ -1,5 +1,5 @@
 {
 	"inherits": ["esp32"],
 	"build-tags": ["m5stack"],
-	"serial-port": ["acm:10c4:ea60"]
+	"serial-port": ["10c4:ea60"]
 }