LU-17696 llite: remove LASSERT from ll_ddelete()

LU-17696 llite: remove LASSERT from ll_ddelete()

On Linux kernel 6.8, the changes in commit 2f42f1eb9093
("Call retain_dentry() with refcount 0") made d_delete()
instances called for dentries with ->d_lock held and
refcount equal to 0, which caused the following assertion
failure on Lustre client:

(dcache.c:136:ll_ddelete()) ASSERTION( d_count(de) == 1 ) failed

The value of d_count(de) became 0 instead of 1. Since
retain_dentry() was called either with refcount 0 or 1,
we can simply remove the LASSERT(ll_d_count(de) == 1)
from ll_ddelete() to avoid the above failure.

Change-Id: Ic4a39d9328326634190cd0719b4c0637e1bf315c
Signed-off-by: Jian Yu <yujian@whamcloud.com>