windows-nat.c: Don't install a deprecated_xfer_memory method.

windows-nat.c: Don't install a deprecated_xfer_memory method.

This stops another target from installing a
target_ops->deprecated_xfer_memory method.

Tested on native MinGW.

gdb/
2013-08-27  Pedro Alves  <palves@redhat.com>

	* windows-nat.c (windows_xfer_memory): Adjust prototype to follow
	xfer_partial's interface.  Return TARGET_XFER_E_IO on error.
	(windows_xfer_partial): Defer TARGET_OBJECT_MEMORY handling to
	windows_xfer_memory directly.
	(init_windows_ops): Don't install a deprecated_xfer_memory method.