Remove racy assert. The thread which changes vnode usecount from 0 to 1
does it under the vnode interlock, but the interlock is not owned by the
asserting thread. As result, we might read increased use counter but also
still see VI_OWEINACT.

In collaboration with: nwhitehorn
Hardware donated by: IBM LTC
Sponsored by: The FreeBSD Foundation (kib)
Approved by: re (gjb)