targets: Remove target. prefix from ARM-CM3DS-MPS2 override

This is the only target in targets.json that uses the `target.` prefix
when defining a configuration override. This looks like an error, and
causes issues with mbed-tools, so this commit removes the prefix.

Author: rwalton-arm

targets/targets.json

@@ -4777,7 +4777,7 @@
         "copy_method": "mps2",
         "reset_method": "reboot.txt",
         "overrides": {
-            "target.network-default-interface-type": "ETHERNET"
+            "network-default-interface-type": "ETHERNET"
         },
         "detect_code": [
             "5004"