Remove the unused PrismMacroLayout.Constants class

The public org.xwiki.contrib.prismjs.PrismMacroLayout.Constants class (holding the PLAIN_HINT and LINENUMBERS_HINT string constants) is not used anywhere: the PrismMacroLayout enum stored these hints in a private field that was never read, and the layouts are not implemented as components that would need such hints in an annotation.

This class and the corresponding unused private hint field of the enum are removed. The PrismMacroLayout enum values (PLAIN, LINENUMBERS) are unchanged, so the layout parameter of the prism macro is not affected.

Backward compatibility

This removes a public class from the API. It is not used by any known extension, so it's removed without a deprecation phase.