ox-html.el: Partially revert
d5bbf36553
* lisp/ox-html.el (org-html-src-block): Unless klipse is used, export
source code blocks as content of `pre' elements, and not as content of
`code' children of `pre' elements. Restores the previous way of
exporting source code blocks, and fixes the display of language
indicators while hovering over the blocks in the exported HTML file,
when klipse is not used.
(org-html-keep-old-src): Remove it. Not needed because it is now the
default, unless klipse is used.