Remove arguments from `webpaste-paste-buffer-or-region`

[8dcc]

Using (interactive "r") produces the following error message right after starting Emacs (or after creating new buffers, perhaps?):

execute-extended-command: The mark is not set now, so there is no region

Since this commit changes the function arguments, it might break some existing calls from Elisp, so the change might not be worth it.

I decided to separate this PR from #61, since they are not related.