openjdk18: disable JavaFX by default

JavaFX is an optional component of Java since version 11, not used by
most applications and also a source of many security issues (i.e.: it
bundles both media codecs and WebKit). Also, it is only available in
some platforms.

So this commit will disable JavaFX by default, that will allow us to
reduce the closure size significantly and reduce the attack surface of
Java applications for most users. Derivations that needs it can always
override the parameter themselves.