Stop forcing IJ.getClassLoader() to return a FijiClassLoader

Now that ij-legacy learnt to force ImageJ 1.x to look for legacy plugins
in $HOME/.plugins/ (or if the system property 'ij1.plugin.dirs' is set,
the directories specified by said property), it is time to retire the
FijiClassLoader.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Deprecate the FijiClassLoader

It served us well...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>