You can't do it inside Acrobat, but there is a way to do it outside of Acrobat. Export the comments as a data file. I wrote about there here.
Change the extension from FDF to XML and open it up in your text editor of choice. Then delete two bits of code. Look for the two sections that have the dates in them. If your have a bunch of comments, it might be easer to do a Search for the year in order to make sure you find them all.
Save the XML file. Change the extension back to FDF. Delete all your old comments (save a backup copy first!) Then reimport it back into your PDF. The dates should be gone. Here is the original comment with the date at the bottom.
And here is the comment after the code was edited in Text Wrangler. No date!