Remove declaration of unlinkat from <fcntl.h>.

Glibc doesn't expose unlinkat in fcntl.h, this probably came about
because the man page for unlinkat mentions `#include <fcntl.h>` for the
AT_* constants that actually are defined there.

Bug: http://b/28067717
Change-Id: I769ca792a189bfea212b1d75096274acb0b7cfdb