Update JavaDoc which references
FredPluginFCPMessageHandler.FCPPluginMessage constructors to mention
shortcuts to them

- The JavaDoc at constructRawMessage() which enumerates all other
constructor functions is removed: Its unnecessary bloat, an average
reader will probably smart enough to check for other constructors on his
own.
- I have also checked the callers of the non-shortcut constructor,s
there are none suitable for replacement. So the job of introducing
the shortcuts is complete, and therefore this also removes the FIXME
of adding the shortcuts.