vfs: delete vfs_readdir function declaration

vfs_readdir() was replaced by iterate_dir() in commit 5c0ba4e0
("[readdir] introduce iterate_dir() and dir_context").

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>