Remove the fallback argument into a constant

!29474 (diffs, comment 335693898)

Is the fallback ever set? I cannot see it used. Nor can I see it tested. I'm inclined to think that if the method is used consistently for counting things in redis then we'll probably always be happy for it to return -1 as a fallback since this is always a good choice for a missing count. Further it seems that the underlying method calls imply this is only used for counts so I'd suggest we remove this argument. It would also be nice to remove because missing different styles for optional arguments in this way is kinda hard to read so it will be much cleaner without that argument.