Sorry, I have been on vacation the last week and haven’t been on top of forum posts. If you google ‘SMD PTC fuse’, they look like this:
http://www.sellifuse.com/pptc/imges/2016_big.jpg
on OSPi it should be just under the LM2596 (or AP1501) switching regulator, should be easy to identify. This fuse connects the output of the switching regulator to RPi’s 5V line. If you remove the fuse, you can power it with microUSB and then the two sources won’t compete with each other.
It’s probably ok if you don’t remove it: I don’t think anything bad will happen. Normally when you have two regulated sources correctly directly to each other, one will dominate.