Do not assert seal transition on forward path

Do not assert seal transition on forward path

Unlike the replay path, where we expect no successor, the forward
path may very well encounter a successor being present.

Move the assertion to replay path only, as that is where it catches
issues. The forward path automatically propagates the seal via its
use of sendRequest(), which is forwarded as needed and therefore
we only note the race via LOG.debug() and not assert it.

Change-Id: I6f95acc49800d456c049d19982c801015fd69420
JIRA: CONTROLLER-1885
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>