Remove leftover dentry variable after hypfs refactoring.

Before 2fcb3686, hypfs_diag.c and other hypfs files were using
debugfs_create_file() explicitly for creating debugfs files and
were storing the returned pointer.

After the refactor, common debugfs file operations and also the
related dentry pointers have been moved into hypfs_dbfs.c and
redefined as new common mechanisms.
Therefore the dentry variable and the debugfs_remove() function
calls in hypfs_diag.c are now redundant.

Current code is not effected since the dentry pointer in
hypfs_diag is implicitly assigned to NULL and debugfs_remove()
returns without an error if the passed pointer is NULL.
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>