Remove User::getPageRenderingHash, deprecated since 1.17

The function is not used in core or in any extension in git,
and the PageRenderingHash hook in User::getPageRenderingHash
is not used in any extension. (note the hook still exists
in ParserOptions).

Change-Id: Ica27ce3828dd6551e21671ff723427d09c2dcadb
Merge "Remove User::getPageRenderingHash, deprecated since 1.17"