Duplicating collections in Linked mode doesn't maintain camera/speaker/
lattice object data, but duplicates it.

Caused rBad6cccf058d0: Refactor duplicate of data-blocks..

Since above commit, object duplication was called with other dupliflags
(USER_DUP_OBJECT was added), so checking if flags were plain 0 was not
working anymore to determine if object data should be linked. This wasnt
a problem for other object types since these have dedicated flags which
camera/speaker/lattice dont have (and these were _not_ set, so
duplication didnt happen there).

Now be permissive with the USER_DUP_OBJECT flag and still consider this
as fully linked in terms of object data.

Also remove check (early return above makes it redundant).