Remove ui_out_destroy

Remove ui_out_destroy

It's not actually used, and removing it simplifies the upcoming patches
a bit.  After the whole series, destroying an ui_out object will be
simply "delete uiout", which will call the default destructor.

gdb/ChangeLog:

	* ui-out.c (ui_out_destroy, uo_data_destroy): Remove.
	* ui-out.h (ui_out_destroy): Remove.