FCPInterface: Remove obsolete comment

This was from the days when the if() two lines below not only checked
the insert URI for being null but also the request URI. Nowdays,
creation of an OwnIdentity from an URI only requires the insert URI, the
comment can be removed.
FCPInterface.handleCreateIdentity(): Synchronize

This is a workaround for the following issue:
https://bugs.freenetproject.org/view.php?id=6247

Synchronize around usage of the OwnIdentity object because it is stored
in the database and thus subject to 6247.
Notice: The removed comment about preserving the locking order didn't
make any sense, it was probably obsolete - we only take one lock here,
so there is no order.
FCPInterface.handleGetTrust(): Document why we synchronize

This commit would have added synchronization similar to the preceding
commits, but the code already contains the necessary synchronization.
Thus, only add a comment which explains why it is there.