Remove tui_clear_source_content

Remove tui_clear_source_content

tui_clear_source_content is not needed.  Instead, the callers can call
erase_source_content, which is also changed to clear the content
vector.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_clear_source_content): Don't declare.
	* tui/tui-winsource.c (tui_update_source_window_as_is): Don't call
	tui_clear_source_content.
	(tui_clear_source_content): Remove.
	(tui_source_window_base::do_erase_source_content): Hoist call to
	content.clear().
	* tui/tui-stack.c (tui_show_frame_info): Don't call
	tui_clear_source_content.