Remove some unnecessary focus switches

A couple of lower-level utility functions can change the TUI focus.
This seems incorrect to me -- focus switches should only be done
either by explicit user request, or ass a side effect of changing the
layout.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c
	(tui_source_window_base::update_source_window_as_is): Don't switch focus.
	* tui/tui-disasm.c (tui_show_disassem): Don't switch focus.

Change-Id: I0a5bb8a407cf8d52e2fd23b0598eb9bce56b1251