[CIRCT] Remove CombinationalPath Anno Dropping (#2996)

Change the CIRCT phase to no longer drop the CombinationalPath
annotation.  This dropping behavior was done to enable an SFC->MFC
handoff where the SFC can produce this annotation.  Chisel can never
produce this annotation, so it is safe to have MFC error if it sees it.

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