(message-hide-headers): Don't bind `inhibit-modification-hooks`

(message-hide-headers): Don't bind `inhibit-modification-hooks`

This binding dates back at least to 2004 (where we bound
`after-change-functions` instead).  Maybe it had a real impact in the past,
but nowadays I can't see any effect other than maybe a negligible
performance improvement at the cost at breaking some uses of
`before-after-change-functions`.

* lisp/gnus/message.el (message-hide-headers):
Don't bind `inhibit-modification-hooks`.

(cherry picked from commit 10802fb00f59594fa8e524e836eaf0e99e87dec9)