device: Remove unnecessary z_impl_device_is_ready()

This is a follow up to commit 2aab687270a9c094591568cc1a6d5d4d436ccd7c.

Since device_is_ready() is no longer a system call, there is no need
to keep z_impl_device_is_ready().

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
1 file changed