ofproto-dpif-upcall: Simplify revalidator_sweep__().

ofproto-dpif-upcall: Simplify revalidator_sweep__().

Broadly, there are two cases that are handled during revalidator_sweep__:
- Ukeys which had their corresponding datapath flows deleted during the
  most recent dump phase need to be deleted.
- If a flow for a ukey still exists in the datapath, the flow may need
  to be removed or updated. This depends on a variety of factors such as
  whether the datapath is being flushed, whether individual flows were
  recently dumped, and whether those flows are valid for the current
  revalidation generation.

Previously, the logic was written such that the first of these cases
would be handled under the "UKEY_KEEP" case to ensure that
revalidator_sweep__() will not attempt to delete flows that already
exist. In this case, ukey->flow_exists would be false, which would
trigger ukey cleanup.

While correct, this is misleading and difficult to follow. Since commit
83b03fe05e7a ("ofproto-dpif-upcall: Avoid double-delete of ukeys."),
this logic is no longer required to prevent double-deletion of such
flows, so we can now make this codepath more straightforward.

Signed-off-by: Joe Stringer <joe@ovn.org>
Co-authored-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>