emu: Remove LValue implementation for u16

I think this was from when registers were passed by mut ref and this
could update them in place. Now they are wrapped and those wrappers
are LValues instead.

src/emu/operands.rs

@@ -48,12 +48,6 @@ impl RValue<u8> for u8 {
   }
 }
 
-impl LValue<u16> for u16 {
-  fn write(&mut self, val: u16) {
-    *self = val;
-  }
-}
-
 impl RValue<u16> for u16 {
   fn read(&self) -> u16 {
     *self