Remove some checks of .empty()

Remove some checks of .empty()

A few methods in tui_data_window check whether the contents are empty;
but all the callers already check this, so these calls can be removed.

gdb/ChangeLog
2019-08-30  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.c (tui_data_window::last_regs_line_no)
	(tui_data_window::display_registers_from)
	(tui_data_window::display_reg_element_at_line)
	(tui_data_window::display_registers_from_line): Remove checks of
	"empty".