Delete def_uiout

Delete def_uiout

Currently, current_uiout starts out pointing to def_uiout, a dummy
ui_out implementation.

Since we create a replacement uiout early on as soon as we create the
interpreter, we never actually use def_uiout.  So this patch removes
it.

The proof that it works is that starting with current_uiout set to
NULL does not crash.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* ui-out.c (default_ui_out_impl): Delete.
	(def_uiout): Delete.
	(current_uiout): Set to NULL.
	(default_table_begin, default_table_body, default_table_end)
	(default_table_header, default_begin, default_end)
	(default_field_int, default_field_skip, default_field_string)
	(default_field_fmt, default_spaces, default_text, default_message)
	(default_wrap_hint, default_flush, default_data_destroy): Delete.