Remove tui_source_window::content_in_use

Remove tui_source_window::content_in_use

Now that source window clearing has been simplified, we don't need a
special flag to say whether the source window is in use -- we can
simply check whether the contents are set.  This patch implements this
idea, removing the content_in_use field.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_clear_source_content)
	(tui_show_source_content): Update.
	* tui/tui-source.c (tui_source_window::showing_source_p): Check
	whether content is empty.
	* tui/tui-data.h (struct tui_source_window_base) <content_in_use>:
	Remove.