SynthLookAndFeel.load(URL url) is not used by any of the JDK's built-in Swing L&Fs
since instead these use the alternate way of loading synth styles via setting
a SynthStyleFactory on the SynthLookAndFeel.

In addition there exists
javax.swing.plaf.synth.SynthLookAndFeel.load(InputStream input, Class<?resourceBase>)
which can be used in migration of any external L&F which loads the styles from a synth resource file.

In addition it is not clear to me how this can easily be used by a 3rd party L&F except if
the URL is relative to the code since it would be extremely fragile to point to an absolute
external URL .. nor can I imagine any consumer of the L&F wanting that it.
Nor are many (probably zero) applications likely to be providing their own.
This allows us to remove this method which does not have an easy way to verify the URL as trusted.