Don't call tui_show_source from tui_ui_out

Don't call tui_show_source from tui_ui_out

This removes the call to tui_show_source from tui_ui_out.  This always
seemed like a hack, and now that the TUI is using the proper
observers, it seems not to be needed.

The rest of the logic remains, unfortunately, because it is needed to
suppress some gdb output in the TUI case.  We could probably find a
nicer way to do this (maybe a ui_out_flag), but I haven't attempted
this.

This was the last caller of tui_show_source, so this is removed as
well.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_show_source): Remove.
	* tui/tui.h (tui_show_source): Don't declare.
	* tui/tui-out.c (tui_ui_out::do_field_string): Don't call
	tui_show_source.

Change-Id: Id71098e597ee4ebfef0429562baa45f537bd2c2b