how can I fix an uneven hydraulic disk brake set I have a cargo bike (http://www.christianiabikes.com/en/product/26/) with front wheels having bengal helix 7.5 hydraulic disk brakes.

Inexperienced me, I have taken off the caliper from the rotor on the right side and squeezed the lever. After that I wasn't able to put the rotor back. Then I took apart the caliper (the entire thing, not just the bleeding valve) and leaked some fluid so that I could put the rotor back and took the bike to the bike shop.

They have filled the liquid up but the brakes became uneven. Bike is steering towards left when it is braked. They are saying that they cannot do any better than that since I messed with it and asking me to buy a new brake set.

Is it really true that brakes are ruined? Can I not fix it by completely disposing the fluid and starting over?
In my view its unlikely the entire brake needs replacing, although if you have to pay someone for the time needed to resolve the problem, its entirely possible it will be the cheapest solution in the end. 

As already said in comments, the most likely cause is contamination of discs and pads with brake fluid. If the pads have been cleaned, the next step is to replace them and thoroughly clean the disk.  

Another possibility is the shop replaced the pads with new ones. In this case there are a couple of possibilities. It may be the new pads just need to be bedded in, and the problem will resolve itself with a half dozen high effort stops. You may also need to replace the pads on the other side with a matching set as different pad material have different characteristics.