As a postdoc, should I expect my PhD adviser to fund my conference travel to present my PhD work?
One should always figure out the funding issues before submitting an abstract to a conference. Presenters withdrawing from a conference are a major headache to the conference organizers and it is unreasonable to expect them to reorganize the conference program because you did not figure out your travel plans (either timing or funding) in advanced. There are cases where there is nothing you can do (e.g., having a child), but in general when you submit an abstract you are committing yourself to presenting the work.

As to who should fund your travel, the best would have been to ask when you submitted to the conference. There really isn't a standard for who should pay. The first issue is to determine if your PhD advisor or Post Doc Advisor has money that could be used to pay for your travel. They would have to have money that could, if they desired, be used to fund your travel, and be willing to fund your travel.

I would ask both people and see what they say. No one is going to be offended by you asking. I wouldn't be surprised if both said no, but nor would I be surprised if both said yes.