Remove tui_source_window::show_symtab_source

Remove tui_source_window::show_symtab_source

tui_source_window::show_symtab_source is identical to
update_source_window, so remove the former.

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

	* tui/tui-winsource.c (tui_update_source_windows_with_addr)
	(tui_update_source_windows_with_line): Call update_source_window.
	* tui/tui-source.h (struct tui_source_window)
	<show_symtab_source>: Don't declare.
	* tui/tui-source.c (tui_source_window::show_symtab_source):
	Remove.

Change-Id: I41781df2126e8bafad46d058532d52602a288e06