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;

src/nvim/os/unix_defs.h

@@ -2,7 +2,7 @@
 #define NVIM_OS_UNIX_DEFS_H
 
 // Windows doesn't have unistd.h, so we include it here to avoid numerous
-// instances of `#ifdef HAVE_UNISTD_H'.
+// instances of `#ifdef WIN32'.
 #include <unistd.h>
 
 // POSIX.1-2008 says that NAME_MAX should be in here

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>