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/log.c

@@ -10,10 +10,6 @@
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"
 
-#ifdef HAVE_UNISTD_H
-# include <unistd.h>
-#endif
-
 #define USR_LOG_FILE "$HOME" _PATHSEPSTR ".nvimlog"
 
 static uv_mutex_t mutex;