[codegen] Simpler FIRRTL Code for Reg w/o Reset (#3280)

Change FIRRTL emission to use a simpler syntax for registers without a
reset.  Previously, this used a self-reset with a zero reset signal.
Simplify this to emit a register without a reset.

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