Reference "FredPluginFCPMessageHandler.FCPPluginMessage" by
"FCPPluginMessage" instead

- It was prefixed because there was a second class with the same name
(freenet.node.fcp.FCPPluginMessage). That duplicate was removed by
commit 667f74ed462155e50dcebf16314341b7577552a8, so now we can remove
the prefix.
- More commits to fix this in more files will follow
Reference "FredPluginFCPMessageHandler.FCPPluginMessage" by
"FCPPluginMessage" instead

- It was prefixed because there was a second class with the same name
(freenet.node.fcp.FCPPluginMessage). That duplicate was removed by
commit 667f74ed462155e50dcebf16314341b7577552a8, so now we can remove
the prefix.
- This is the last commit in the series to fix this everywhere. Thus, it
removes the relevant FIXME from the code.