depends: remove PYTHONPATH from config.site

We no-longer need this, as we no-longer build python packages.

Effectively reverts de619a37fd.

depends/config.site.in

@@ -102,7 +102,6 @@ fi
 if test -n "@CXX@" -a -z "${CXX}"; then
   CXX="@CXX@"
 fi
-PYTHONPATH="${depends_prefix}/native/lib/python3/dist-packages${PYTHONPATH:+${PATH_SEPARATOR}}${PYTHONPATH}"
 
 if test -n "@AR@"; then
   AR="@AR@"