Remove unnecessary historical note from "Object storage format"

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -4145,8 +4145,6 @@ about the data in the object.  It's worth noting that the SHA-1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
-(Historical note: in the dawn of the age of Git the hash
-was the SHA-1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can