How can I remove a signature once it's saved in

A possibility is to try
File > Revert To > Browse all versions...

though I guess it will depend on how far back that version history goes. I've been unable to discover just how far back that would be & for how long it may be saved.

You cannot remove a signature in Yosemite. This is a bug. It would almost make sense if signing a document froze the entire document,but you can edit every other annotation.

I've tried masking a signature with a filled box, then overlaying a signature on that. It seems to work and the document saves, but on opening tne new signature is gone. (Another bug.)

I was able to do it by using a rectangular annotation via Tools -> Annotate -> Rectangle. Make sure it covers the signature and just make the background and border match the color of your document background (white in my case).

Even on Mojave it still seems to be the case that once you have saved the file and re-opened it, you will not be able to remove the image.

To resolve this I followed the instructions in Use Preview to combine PDFs on your Mac to add a copy of the blank signature page from the original to my modified document. I was then able to edit and sign that newly-added page and so complete the document.