[JumpThreading] Remove extraneous calls to setEdgeProbability

This patch removes extraneous calls to setEdgeProbability introduced
in c9148776.

The follow-up patch, a7b662d0, has
since fixed BranchProbabilityInfo::eraseBlock, so we don't need to
worry about getting stale values from getEdgeProbability.

Also, since getEdgeProbability(BB, BB->getSingleSuccessor()) returns
edge probability 1/1 by default for BB with exactly one successor
edge, we don't need to explicitly call setEdgeProbability.

This patch introduces almost no functional change, but we do end up
reducing debug messages from setEdgeProbability.

Differential Revision: https://reviews.llvm.org/D90284