Remove useless "if' from tui-regs.c

tui_data_window::show_register_group had a useless "if" -- the
condition could never be false.  This patch removes it.

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

	* tui/tui-regs.c (tui_data_window::show_register_group): Remove
	useless "if".