glob: backslash escaping bug

curl didn't properly handle escaping characters in a URL with the use of
backslash. It did an attempt, but that failed as reported in bug
3022551. The described example was using the URL
"http://example.com?{AB,C\,D}".

I've now removed the special-handling of letters following the backslash
and I also removed the bad extra check that triggered this particular
bug.

Bug: http://curl.haxx.se/bug/view.cgi?id=3022551
Reported by: Jon Sargeant