hfsplus: use xattr handlers for removexattr

hfsplus was already using the handlers for get and set operations,
and with the removal of can_set_xattr we've now allow operations that
wouldn't otherwise be allowed.

With this we can also centralize the special-casing of the osx.
attrs that don't have prefixes on disk in the osx xattr handlers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>