ethernet: temporary mitigation for test that uses TUNSETCARRIER

Since creating an interface without carrier can still have carrier for a
short period of time, the never available assertion is flaky.

This change removes the assertion which is to be readded after
IFF_NO_CARRIER is supported in the kernel.

Test: atest EthernetManagerTest
Bug: 249611919
Change-Id: I437fa2ed34fa28c07b66358b63be40a80d00acf0