Fixes an issue with FIP re-association

When the last FIP is disassociated, the namespace and
interfaces should be removed. The internal interface
wasn't removed before without problems, but now the
namespace cannot be removed with that interface present.
The fix is to remove the internal FIP interface before
removing the namespace.

Change-Id: I021c658ecde584821f67b7a8de0205e8e938bb2d
Closes-bug: 1353287
Merge "Fixes an issue with FIP re-association"