pinmux: Remove the k64 pinmux driver

The k64 pinmux driver was deprecated when the more generic mcux pinmux
driver was added, but it can actually just be removed because it is not
a public interface. Applications should be using the public pinmux API,
not the private k64 pinmux API. There was one case in the net samples
that used the private API which was cleaned up in a previous patch.

Change-Id: I49a6397baa57973930cb63bd2a9883b14f7ddafd
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>