Remove the unused _validate_device_name()

Since the commit I898ae56539c6101680983d1eca4bce6d92bd5ce6,
_validate_device_name() method has been unused.
This patch removes the method.

Change-Id: I642b71385432801eb8fdd35484fbcb3ed5efa795
Merge "Remove the unused _validate_device_name()"