Why is the battery jumpered to a resistor in this schematic?
I don't know what this shield looks like, but I'm going to guess that the battery is removable/replaceable.  Is that the case?  

Did it come with a battery, or was it up to you to provide it?  

The DS1307 has a little quirk in that it requires that Vbat be grounded when there is no backup battery connected.  R5 is meant to pull Vbat down to ground in the absence of a backup battery.  Without that resistor, the DS1307 would not function correctly if one were to try and use it without a battery installed and powered via the ESP mainboard/VCC.  

From the DS1307's data sheet: 


  If a backup supply is not required, VBAT must be grounded.


Now, is it a good solution to this requirement?  ...Not really.  For a 48mAh 3V cell, this will reduce the best possible backup time to 3 years, and the drain will occur even when the DS1307 is running off VCC.  

However, I can definitely understand their reasoning.  It's a shield, it is aimed at a fairly wide audience and they probably just made the decision that it would be better if the DS1307 worked in every possible usage case rather than keep time for a decade.  

That said, as long as you aren't planning on using it without a backup battery installed, you can simply remove it and eliminate that source of drain on the battery.