Remove Vwindow_text_change_functions and related code

Remove Vwindow_text_change_functions and related code

Vwindow_text_change_functions had been provided for implementing
line numbers but apparently was never functional or in use.

* src/xdisp.c (redisplay_window): Remove handling of
Vwindow_text_change_functions.
(syms_of_xdisp): Remove Qwindow_text_change_functions.
(Vwindow_text_change_functions): Remove variable.
* doc/lispref/hooks.texi (Standard Hooks): Remove entry for
`window-text-change-functions'.