The patch files themselves that you downloaded via blizzard downloader will be in the folder you installed your game in. They will either be under the "patches" subfolder or just be in the main folder as individual files.

Deleting these make no difference at all to game play because all the changes have been made to your .mpq files.

If you have been playing since BC, the patches can easily reach multiple gigabytes, so that's a great idea.

EDIT: Liam, the autopatcher for WoW downloads a patch mpq onto the hard drive. This varies in size from 60mb to 1.4 GB. The updater then uses the file data to patch your game files, such as "textures.mpq" and other game files.

There is no way you can delete a patch of information from the game MPQs themselves. Once you patch, the game couldn't care less about what happened to the patch MPQs. If you copy and paste a joke from a word document to the internet, the joke doesn't disappear from the internet when your word document gets deleted. Same deal with patch mpqs, they're used to copy some stuff over and just left to sit there.