[otbn,dv] Remove static is_xz function from otbn_model.cc

The last use of this was removed in 394ce05

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/dv/model/otbn_model.cc b/hw/ip/otbn/dv/model/otbn_model.cc
index 0255d58..cbba08b 100644
--- a/hw/ip/otbn/dv/model/otbn_model.cc
+++ b/hw/ip/otbn/dv/model/otbn_model.cc
@@ -111,8 +111,6 @@
   }
 }
 
-static bool is_xz(svLogic l) { return l == sv_x || l == sv_z; }
-
 template <typename T>
 static std::array<T, 32> get_rtl_regs(const std::string &reg_scope) {
   std::array<T, 32> ret;