boards: frdm_k64f: remove as default platform

With the amount of coverage we are getting through qemu and other
simulators on ARM and other architectures, no need to have a
`build-only` board running in CI.

This was here for legacy reasons and due to the fact it was the first
board to be supported by zephyr.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

boards/arm/frdm_k64f/frdm_k64f.yaml

@@ -26,5 +26,3 @@ supported:
   - spi
   - usb_device
   - watchdog
-testing:
-  default: true