clean out unused code in dentry pruning

It looks like in the end all pruners want parents removed.

So remove unused code and function arguments.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>