[otbn] Get rid of unused backdoor argument in ISS Reg write

I'm not quite sure why I added this, but we don't use it anywhere, so
let's get rid of it.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

hw/ip/otbn/dv/otbnsim/sim/gpr.py

diff --git a/hw/ip/otbn/dv/otbnsim/sim/gpr.py b/hw/ip/otbn/dv/otbnsim/sim/gpr.py
index 61cb90b..b08243c 100644
--- a/hw/ip/otbn/dv/otbnsim/sim/gpr.py
+++ b/hw/ip/otbn/dv/otbnsim/sim/gpr.py
@@ -48,9 +48,6 @@
         self.saw_read = True
         return self.stack[-1]
 
-    def write_unsigned(self, uval: int, backdoor: bool = False) -> None:
-        super().write_unsigned(uval, backdoor)
-
     def commit(self) -> None:
         if self.saw_read:
             assert self.stack

hw/ip/otbn/dv/otbnsim/sim/reg.py

diff --git a/hw/ip/otbn/dv/otbnsim/sim/reg.py b/hw/ip/otbn/dv/otbnsim/sim/reg.py
index ad102db..c8ba04c 100644
--- a/hw/ip/otbn/dv/otbnsim/sim/reg.py
+++ b/hw/ip/otbn/dv/otbnsim/sim/reg.py
@@ -41,10 +41,10 @@
     def read_unsigned(self, backdoor: bool = False) -> int:
         return self._uval
 
-    def write_unsigned(self, uval: int, backdoor: bool = False) -> None:
+    def write_unsigned(self, uval: int) -> None:
         assert 0 <= uval < (1 << self._width)
         self._next_uval = uval
-        if not backdoor and self._parent is not None:
+        if self._parent is not None:
             self._parent.mark_written(self._idx)
 
     def read_next(self) -> Optional[int]: