Remove dead code from TetherInterfaceSM

( cherry-pick of 2ea4166c )

Because no code outside of unittests ever issued these commands
to TetherInterfaceSM, we never executed any of the removed codepaths.

Change-Id: Id54f6e4eaeff8b3486cd78ddcc8c2a31011e6436
Test: Compiles.  Unittests continue to pass.
Bug: 28798823