[BOLT][NFC] Remove misleading debug message

Summary:
The debug message for the last fall-through block was printed under the
reverse condition, i.e. when the block was not a fall-through. Remove
the debug message. If we'll need such information, we can add a pass
with more analysis, i.e. checking the last instruction, if the block is
reachable, etc.

(cherry picked from FBD32670816)