Remove obsolete class references

And do not mention SezPoz explicitly, since discovery is taken care of
by the SciJava Common framework in an unspecified way, which SCIFIO does
not need to care about anymore!
fixup! Corrected how filters are indexed
Use ClassUtils#loadClass instead of Class#forName

The loadClass method uses the current thread's context class loader,
rather than the system class loader, by default, which is much more
robust in situation's like Fiji's.