Use readlink unconditionally

Use readlink unconditionally

Since readlink module is imported, we can use it unconditionally.
This patch is to remove configure checks and HAVE_READLINK checks in
code.  It was mentioned in the patch below

  [RFA/commit] gdbserver: return ENOSYS if readlink not supported.
  https://sourceware.org/ml/gdb-patches/2012-02/msg00148.html

to use readlink in gdbserver, but we chose something simple at that
moment.

gdb:

2014-11-28  Yao Qi  <yao@codesourcery.com>

	* configure.ac (AC_CHECK_FUNCS): Remove readlink.
	* config.in, configure: Re-generate.
	* inf-child.c (inf_child_fileio_readlink): Don't check
	HAVE_READLINK is defined.

gdb/gdbserver:

2014-11-28  Yao Qi  <yao@codesourcery.com>

	* configure.ac(AC_CHECK_FUNCS): Remove readlink.
	* config.in, configure: Re-generate.
	* hostio.c (handle_unlink): Remove code checking HAVE_READLINK
	is defined.