Tamsulosin won't be having the intended effect as all the tissue it acts on has gone.
However, it's also a blood pressure reducing medication, so if you stop taking it, you need to monitor your blood pressure which may rise and might need a different treatment. Having said that, general anesthetics can lower blood pressure for a week afterwards, so bare that in mind too.
Finasteride acts on the prostate, and if that's why you were on it, then that's not needed any more either. Also, it messes with PSA tests, so it might be a good idea to be off it for as long as possible before your first post-op PSA test.
As always, discuss medication changes with your clinicians.