Open up your character information window (hotkey "C") and click on the tab at the bottom that says "skills"
Scroll down till you find Alchemy and Herbalism (they are two separate skill bars). There will be a circle with a line through it (think of an elementary school no smoking sign or a cheap picket sign). Click on that.
It will then ask for confirmation of if you really wish to unlearn that skill. Click yes (or okay, whatever one it is)
You should be good to go! =D