zephyr-kernel: don't call zephyr_library() twice

The patch enabling the ACM0 console adds a call to zephyr_library() that
with certain Kconfig option can be called twice leading to build failures.
Remove the duplicate.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>
Suggested-by: Eilís Ní Fhlannagáin <elizabeth.flanagan@huawei.com>