unfork navigateReducer (#68816)
In #68815 I updated the only part that differed between the ppr/non-PPR
router handling to rely on a flag that could only be set by a PPRed
route.
Since nothing else is different between the two reducers aside from this
block, this removes the PPR fork. If a route is PPRed and postpones, it
will flow into the PPR handling.
Closes NDX-179
