HVAC leaking from saturation
Two long for a comment:
Never call a plumber for an hvac issue. Without a trap the air handler may pull a vacuum on the drain line. The line worked at one time so it is probably blocked this is a daily issue for the residential guys, if the coil, pan and drain line were not properly cleaned it’s time to get it clean or the problem will return. Now look at that coil on the air handler if you are cooling your home and there is any humidity guess what, that coil will start producing water we call it condensate. It is how the unit works, now that the trap has been removed make sure not to turn the fan on while in cooling mode or you may have a real flood. Water dripping from the coil is a normal part of how an ac system works, pit a glass of ice water on the counter notice the moisture developing and maybe beads of water running down the glass, now multiply that by a thousand and you have a normal evaporator coil (the one in the air handler).  It is normal to see gallons a day on a central air system.
The big thing is to clean the “growth and dust” out of the coil, pan and drain. Non acid cleaners are the best, 
You may want a hvac tech to set up the trap again, depending on the location these can be 2-3 times as tall as a standard trap to keep the water in the trap.