IJHacker: be less overzealous when setting the context class loader

On MacOSX, we must not set the plugin class loader as the current
thread's context class loader before calling the MacAdapter, or else
things break down rather horribly (as noticed by Jean-Yves Tinevez and
Curtis Rueden, ImageJ would no longer find any plugin classes with
dependencies).

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