Remove some NULL checks from the TUI

Remove some NULL checks from the TUI

I found a few spots in the TUI that were NULL-checking the result of
XNEW.  This cannot return NULL, so this patch removes the checks.

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

	* tui/tui-data.c (tui_alloc_generic_win_info)
	(tui_alloc_win_info, tui_add_content_elements): Remove NULL
	checks.