Strip dependencies from BlackBoxSourceHelper (#1840) (#1841)

Remove all prerequisites and optionalPrerequisites from
BlackBoxSourceHelper. These were false dependencies that were added
without me actually looking at what the transform does.
BlackBoxSourceHelper is an identity transform that only performs IO
side effects based on Chisel-generated annotations. Therefore this
transform can literally run anytime it wants without consequence.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
(cherry picked from commit 4b69baba

)

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>