Remove the svn:externals property that pulls a copy of libcharset

Remove the svn:externals property that pulls a copy of libcharset
into a subdirectory of `halibut'. It wasn't very good anyway (since
it insisted on loading via an unauthenticated svn:// URL). The
Halibut makefile now expects _either_ a subdir `charset', _or_ a
directory called `charset' as a sibling of `halibut', and will work
with the first of those that it finds. A new release script arranges
to provide the former in source tarballs (so that building if you're
an ordinary user is just as simple as it always was).

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4808 cda61777-01e9-0310-a592-d414129be87e