pinetime: update the target file

* Rename pinetime-devkit0 to pinetime because the production device is
    almost the same hardware (the only noticeable difference is a
    different accelerometer, which isn't part of the board file).
  * Remove the UART and set serial to none. The UART uses a lot of
    current by default, so it seems better to disable it.

This is a breaking change, but honestly I think I'm the only one who has
ever actually used TinyGo for the PineTime and I'm fine with this
change :)

targets/pinetime.json

@@ -1,7 +1,7 @@
 {
 	"inherits": ["nrf52"],
-	"build-tags": ["pinetime_devkit0"],
-	"serial": "uart",
+	"build-tags": ["pinetime"],
+	"serial": "none",
 	"flash-method": "openocd",
 	"flash-command": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
 	"openocd-interface": "jlink",