Remove unnecessary "return"s

Remove unnecessary "return"s

The TUI has some "return;" statements at the end of void-returning
functions.  There's no need for these, so this patch removes them.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_update_source_window)
	(tui_update_source_window_as_is)
	(tui_update_source_windows_with_line): Remove return.
	* tui/tui-disasm.c (tui_show_disassem)
	(tui_show_disassem_and_update_source): Remove return.
	* tui/tui.c (tui_reset): Remove return.
	* tui/tui-wingeneral.c
	(tui_check_and_display_highlight_if_needed): Remove return.