Remove overly verbose debug from GenericDomTreeConstruction

This particular debug shows all the time when I'm processing any MLIR
module:

	discovered a new reachable node ^bb0
	discovered a new reachable node ^bb0
	discovered a new reachable node ^bb0
	discovered a new reachable node ^bb0
        ...
        (repeated x1875)

I think that printing all the basic blocks in the function is likely low
value enough that we can get away with removing this.

Differential Revision: https://reviews.llvm.org/D76813