Remove more unnecessary casts of NULL

Remove more unnecessary casts of NULL

I found a few more spots that unnecessarily cast NULL to a pointer
type.  My earlier search included a "*" in the cast, but these use a
typedef to a pointer type instead.  This patch removes these casts.

gdb/ChangeLog
2019-06-16  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.c (tui_clear_win_detail, init_win_info)
	(tui_free_window, free_content, free_content_elements): Remove
	unnecessary cast.
	* tui/tui-windata.c (tui_display_all_data): Remove unnecessary
	cast.
	* tui/tui-regs.c (tui_show_register_group)
	(tui_display_registers_from, tui_display_reg_element_at_line):
	Remove unnecessary cast.