gdb: remove gen_ret_current_ui_field_ptr

gdb: remove gen_ret_current_ui_field_ptr

I think it would be clearer to not use gen_ret_current_ui_field_ptr to
generate the implementation of current_ui_gdb_stdout_ptr et al.  It
doesn't save much code, but adds a layer of complexity for the reader.
Plus, it doesn't work well with IDEs, for example if you ask to find all
usages the m_gdb_stdout field.

gdb/ChangeLog:

	* top.c (current_ui_gdb_stdout_ptr): Spell out by hand.
	(current_ui_gdb_stdin_ptr): Likewise.
	(current_ui_gdb_stderr_ptr): Likewise.
	(current_ui_gdb_stdlog_ptr): Likewise.
	(current_ui_current_uiout_ptr): Likewise.
	(gen_ret_current_ui_field_ptr): Remove.

Change-Id: I86f821c9d119453701caedf0e47124ccddfbab2d