Leaving extra coils on a transformer unconnected
Leaving the winding unconnected will not harm the transformer or do anything else bad. Zero current is acceptable.

However, you may have another problem. 26.3V rectified is 26.3V*1.41=37V, which isover the maximum allowed input voltage for the 7815. Also, unless you plan to only use a few miliamps on the output, the regulator will dissipate a lot of power, so it will need a big heatsink (and a fan possibly).