Remove NO_DATA_STRING

Remove NO_DATA_STRING

NO_DATA_STRING shouldn't be used.  It's referenced in a single spot,
in tui_data_window::display_all_data.  This patch removes the use and
replaces it with the more correct text.  A later patch (though not in
this series) will remove this call entirely, when it's more obviously
correct to do so.

gdb/ChangeLog
2019-08-30  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_data_window::display_all_data): Change
	text.
	* tui/tui-data.h (NO_DATA_STRING): Remove define.