libgfortran: Use remove(3) instead of unlink(2) when deleting files.

libgfortran: Use remove(3) instead of unlink(2) when deleting files.

testsuite:

2015-09-04  Janne Blomqvist  <jb@gcc.gnu.org>

	* gfortran.dg/read_dir.f90: Delete empty directory when closing
	rather than calling rmdir, cleanup if open fails.

libgfortran:

2015-09-04  Janne Blomqvist  <jb@gcc.gnu.org>

	* io/unix.h (delete_file): Remove prototype.
	* io/unix.c (delete_file): Remove function.
	* io/close.c (st_close): Replace delete_file and unlink with
	remove.
	* io/open.c (already_open): Replace unlink with remove.

From-SVN: r227472