Remove the configure and cmake checks for sys/wait.h

Remove the configure and cmake checks for sys/wait.h

If we don't have sys/wait.h and we're on a unix system there's no way
that several of the llvm tools work at all. This includes clang.

Just remove the configure and cmake checks entirely - we'll get a
build error instead of building something broken now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243957 91177308-0d34-0410-b5e6-96231b3b80d8