Dropping a Course in my Field Before Applying to Graduate School
I wouldn't worry about it. You have a perfectly valid reason for dropping. Your advisor can give the best advice, of course, knowing you better than we do, and also knowing other options you might have. 

But a poorly taught course won't give you much of a push toward your goal in any case. 

But if your advisor is "on board" with you dropping and supports you generally, then you should be fine for the more important task of getting good letters of recommendation, which will count much more heavily than a W on the transcript.