gdb: remove pre_init_ui_hook from top.c

This hook appears to be unused.  I guess it was used from insight or
something like that at some point.  But I grepped in today's source of
insight [1] and there was no match.  So I think it's safe to remove.

gdb/ChangeLog:

	* top.c (pre_init_ui_hook): Remove.

[1] https://sourceware.org/git/?p=insight.git

Change-Id: Ia14499a4b6b9d79bb9a526d635fe44a654ef2a27