config: Remove HAVE_UNISTD_H.

Unix systems must have this header but Windows does not have it at all.

Since src/nvim/os/unix_defs.h includes <unistd.h> without the guard
in order to avoid including this in the numerous places we would
need <unistd.h> on Unix we just include src/nvim/os/os.h which will pull
in <unistd.h> for us.

src/nvim/shada.c

@@ -6,9 +6,6 @@
 #include <inttypes.h>
 #include <errno.h>
 #include <fcntl.h>
-#ifdef HAVE_UNISTD_H
-# include <unistd.h>
-#endif
 #include <assert.h>
 
 #include <msgpack.h>