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>