Cleanup plot.bfast

Currently the return value of `plot.bfast` is random and differs depending on the last command run, usually returning a numeric `1` from `par()` which is meaningless. It should not return anything.

Start by locating the plot.bfast implementation and inspect how its final expression determines the return value. Confirm the current behavior, then make the function return nothing and verify that calling plot.bfast no longer yields the incidental numeric value.

Written by the indexing model from the issue text.