Remove the clang/INPUTS directory; NFC

Remove the clang/INPUTS directory; NFC

This directory seems to be unused. At least, when I remove it, I can
still build and all of the lit tests pass for me. I can't find any real
information on why this directory exists in the first place, and the
fact that it hasn't been touched in 10 years (or longer in most cases)
leads me to believe it's safe to remove entirely.