CMake: Don't link libjpeg in MgRenderers

[jng]

I saw this interesting message in making the deb packages:

dpkg-shlibdeps: warning: package could avoid a useless dependency if /tmp/work/build_area/mapguide/mg_deb/debian/mapguidecommon/usr/local/mapguideopensource-4.0.0/lib64/libMgRenderers.so /tmp/work/build_area/mapguide/mg_deb/debian/mapguidecommon/usr/local/mapguideopensource-4.0.0/lib64/libMgRenderers-4.0.0.so were not linked against libjpeg.so.8 (they use none of the library's symbols)

If we're linking to libjpeg, we can remove it. If MgRenderers was the only one trying to link to libjpeg, we can also stop finding it as well