Reuse: Added FixMe for the removal of external references

Issue af3#4346 added a fix to remove external references when elements
are added to a reuse library. This should be removed, as soon as
af3#4150 is implemented.

Issue-ref: 4150
Issue-URL: af3#4150

Issue-ref: 4346
Issue-URL: af3#4346



Signed-off-by: Andreas Bayha <bayha@fortiss.org>