Don't call set_current_source_symtab_and_line from TUI

update_source_window_as_is calls set_current_source_symtab_and_line,
but I don't think there is any reason it should be doing this.  This
patch removes the call.

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 call
	set_current_source_symtab_and_line.

Change-Id: I1152fc7c78150974bd3d555b8568a6f88b65dbe6