[chisel5] Remove FIRRTL backend

Remove the FIRRTL backend.  This does not remove FIRRTL serialization,
just the SFC Transforms that create EmittedFirrtlModule annotations.  A
FIRRTL circuit can still be serialized to text in the normal way.

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