FCPPluginClient: Remove FIXME of testing thread safety

Resolved by the previous commit
(c9f12efd8f09d8a8c2aab2456e876b4c8b181f74), which introduced a unit
test which  attempts to produce race conditions to check whether the
said table leaks.

Notice: I have no idea why the FIXME was at the InterruptedException
section. IMHO it doesn't matter how the function returns as the cleanup
of the table is in a finally{}.