Remove l3dt/xmetadata

While some of the ideas here were perhaps good, the business about objects/more
complex data structures worries me. Rather than leave this stuff around, I'm
happier removing for the present: it will after all be available in the
version control system.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4691 de43f980-851b-0410-b2f7-c40aca1f87e0
Move detail about \tl_to_str:n (etc.) to l3tl

The more detailed version moved here from l3str is useful, certainly in the
context of 'l3in3e'. As the function is clearly \tl_... not \str_..., the
documentation should be in l3tl. Upcoming changes to integrate (some of) l3str
into l3kernel will make this clearer.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4692 de43f980-851b-0410-b2f7-c40aca1f87e0