Revert "Always run javadoc using the ToolProvider API"

Revert "Always run javadoc using the ToolProvider API"

This reverts commit f70aeb5e, except for some commented-out parts of
code and an unused method. I also simplified the code, e.g. with regard
to exception handling, and did some more cosmetic stuff, but basically
it is a revert.

In order to make it compile on more recent JDKs which doe not have class
com.sun.tools.javadoc.Main, I used Class.forName in the method being
called on JDK 8 only.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>