lib/n_d_remove_message: do not remove unique filename

lib/n_d_remove_message: do not remove unique filename

It is wasteful to remove a filename term when the whole message
document is about to be removed from the database. Profiling with perf
shows this takes a significant portion of the time when cleaning up
removed files in the database.

The logic of n_d_remove_message becomes a bit more convoluted here in
order to make the change minimal.

It is possible that this function can be further optimized, since the
expansion of filename terms into filenames is probably not needed
here.