Remove the first_right_arg hack for HMod.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 0b2bf822e847e50f3d9c9c74c36e5a5a93428481..2a734728a28e0df8f5ed32d12557ecf1c620837e 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1469,11 +1469,6 @@ LInstruction* LChunkBuilder::DoMod(HMod* instr) {
               instr->CheckFlag(HValue::kBailoutOnMinusZero))
           ? AssignEnvironment(result)
           : result;
-    } else if (instr->fixed_right_arg().has_value) {
-      LModI* mod = new(zone()) LModI(UseRegister(left),
-                                     UseRegisterAtStart(right),
-                                     NULL);
-      return AssignEnvironment(DefineSameAsFirst(mod));
     } else {
       // The temporary operand is necessary to ensure that right is not
       // allocated into edx.