atktext: remove 'text-update' signal

That signal was created with wrong parameters, it is not used
at all, and it is not clear if it is needed. So removing until
someone brings the topic again, with a valid reasoning. Right now
the only one is being more compatible with IA2.

See also:
https://mail.gnome.org/archives/gnome-accessibility-devel/2013-August/msg00013.html
atktext: Fixing some typos on atk_text_get_text_at_offset deprecation

Also updated atk_text_get_text_[before/after]_offset deprecation
and documentation.
Release 2.9.4