Can I unplug attic fan over garage
Negative pressure in your garage is far more likely to be relieved through flimsy overhead door seals than from inside the adjacent home (presumably through the service door weather seals alone). 

However, it's probably not doing you much economic good to have a fan running in the garage attic. What little energy you save conditioning your home (with its separate attic space) may be offset by the cost of the fan operation. 

If your garage gets uncomfortably warm in the summer it may be worthwhile to run the fan. Otherwise, it's apparently not serving a purpose.