subr.el (inhibit-point-motion-hooks): Mark it as obsolete

subr.el (inhibit-point-motion-hooks): Mark it as obsolete

When Emacs-25 changed the default of `inhibit-point-motion-hooks` to t,
/etc/NEWS declared it as obsolete and so did its docstring, but
it was not marked as obsolete so as not to emit too many warnings
in code where it was impractical to remove the let-binding while
keeping backward compatibility with older Emacsen.
Those backward compatibility issues are not serious any more now that
Emacs-25 is itself considered old.

* lisp/subr.el (inhibit-point-motion-hooks): Mark it as obsolete.
* src/textprop.c (inhibit-point-motion-hooks): Don't mention it being
obsolete in the docstring any more.
* lisp/simple.el (line-move-1, line-move-finish): Silence warnings.