gdb: remove cmd_list_element::pre_show_hook

This is unused, remove it.

gdb/ChangeLog:

	* cli/cli-decode.h (struct cmd_list_element) <pre_show_hook>:
	Remove.
	* cli/cli-setshow.c (do_show_command): Adjust.

Change-Id: Ib9cd79d842550392b062309e1e5c079ad5d7571a