More trivial dead code removal: in int_to_roman(), checking for "num == -1"

More trivial dead code removal: in int_to_roman(), checking for "num == -1"
is redundant after a check has already been made for "num < 0". The "set"
variable can also be removed, as it is now no longer used. Per checking
with Karel, this is the right fix.

Per Coverity static analysis performed by EnterpriseDB.