Remove mergeHyperLogLog.

It's buggy.  If somebody needs this later, they'll need to put back
a non-buggy vesion of it.

Discussion: CAM3SWZT-i6R9JU5YXa8MJUou2_r3LfGJZpQ9tYa1BYxfkj0=cQ@mail.gmail.com
Discussion: CAM3SWZRUOLsYoTT83QgdUy9D8ehYWm_nvbrrfcOOzikiRfFY7g@mail.gmail.com

Peter Geoghegan