Remove recently-added \tl_(upper|lower)case:nn stuff

As discussed in issue #141, there are separate use cases for
the primitives here, and it's likely we want to cover the
'odd category code' case with a name which reflects this.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4481 de43f980-851b-0410-b2f7-c40aca1f87e0