clean up left over html files

[積丹尼 Dan Jacobson]

Here we see a second run of makeinfo --html has only updated the first file:
$ ls -ogt
-rw-r--r-- 1 1714 12-04 23:59 index.html
-rw-r--r-- 1 2757 12-04 23:49 First-Chapter.html
-rw-r--r-- 1 2327 12-04 23:49 First-Section.html
-rw-r--r-- 1 3422 12-04 23:49 Index.html
-rw-r--r-- 1 2334 12-04 23:49 Second-Section.html

That is because I altered the source, making it much simpler, so that
the other files were no longer output.

Well, shouldn't the other files in that directory be cleaned up on the second 
run?

Test with
(info "(texinfo) Short Sample Texinfo File")
on first run, then removing most of the lines for the second run.

OK, maybe it is being extra cautious and that is good.

$ makeinfo --version
texi2any (GNU texinfo) 6.8