Remove tui_data_window::name

The "name" member of tui_data_window was set, but never used.  This
removes it.

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

	* tui/tui-regs.c (tui_data_window::show_register_group): Update.
	* tui/tui-regs.h (struct tui_data_item_window) <name>: Remove.