Remove revalidationEnabled from DnsTlsDispatcher::Transport

The revalidationEnabled field was used to ensure that the two flags
`dot_revalidation_threshold` and `dot_xport_unusable_threshold` are
either set or unset. However, we will need to set
`dot_xport_unusable_threshold` and keep `dot_revalidation_threshold`
unset. So, the field is removed.

Bug: 79727473
Test: set dot_xport_unusable_threshold=1 and ran atest
      set dot_xport_unusable_threshold=-1 and ran atest
Change-Id: Ia074af66f2077e9c32284c8fd027bcadac676343