Remove tui_expand_tabs

tui_expand_tabs only has a single caller.  This patch removes this
function, in favor of a tab-expanding variant of string_file.  This
simplifies the code somewhat.

gdb/ChangeLog
2020-07-01  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_item_window) <content>: Now a
	std::string.
	* tui/tui-regs.c (class tab_expansion_file): New.
	(tab_expansion_file::write): New method.
	(tui_register_format): Change return type.  Use
	tab_expansion_file.
	(tui_get_register, tui_data_window::display_registers_from)
	(tui_data_item_window::rerender): Update.
	* tui/tui-io.h (tui_expand_tabs): Don't declare.
	* tui/tui-io.c (tui_expand_tabs): Remove.