LU-13814 clio: remove discard for transient pages

LU-13814 clio: remove discard for transient pages

With cl_page_delete removed for transient pages, now
cl_page_discard doesn't do anything for transient pages.

Remove it.

Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: Ib62e8a5aea71669428b7b61ba989867702bf1758
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52081
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>