[circt] Remove CIRCTStage optional prereq on SFC (#3023)

Remove an unnecessary optionalPrerequisite of CIRCTStage on the SFC
Compiler phase.  This is entirely unnecessary given that there is no
longer an SFC -> MFC handoff path within CIRCT-based Chisel compilation
flows.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>