LU-10994 clio: remove unused convenience functions

LU-10994 clio: remove unused convenience functions

Remove the unused convenience functions cl_page_top(), cl_page_at(),
cl_page_at_trusted(), and cl2vm_page().

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I9c994d8f4c81bc93383a9eb46def514685a27690
Reviewed-on: https://review.whamcloud.com/47401
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>