Simplify tui_source_window_base::show_source_content

Simplify tui_source_window_base::show_source_content

tui_source_window_base::show_source_content is not used outside the
class any more, so this makes it private.  Examining the callers shows
that it can't be called without source contents, so it can be
simplified as well.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<show_source_content>: Now private.
	* tui/tui-winsource.c
	(tui_source_window_base::show_source_content): Don't handle empty
	content case.