Remove a FIRRTL phase CIRCTStage "currentState" (#3024)

Remove an unnecessary "currentState" argument when constructing the
internal PhaseManager of CIRCTStage.  This is no longer necessary given
that CIRCTState doesn't have to interoperate with SFC in any way.

This enables removal of this phase on the chisel5 branch.

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