Remove tui_clear_exec_info_content

Remove tui_clear_exec_info_content

After the previous patch, all calls to tui_clear_exec_info_content
come just after a call to tui_clear_source_content.  Because these two
windows are linked, I think it makes sense to have
tui_clear_source_content simply do the work.  So, this patch removes
tui_clear_exec_info_content.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_clear_exec_info_content): Don't
	declare.
	* tui/tui-winsource.c (tui_update_source_window_as_is)
	(tui_update_source_windows_with_addr, tui_erase_source_content):
	Update.
	(tui_clear_exec_info_content): Remove.