[AutoDiff] NFC: remove unused variable. (#32411)

Author: dan-zheng

lib/AST/Attr.cpp

@@ -1711,9 +1711,8 @@ void DifferentiableAttr::setParameterIndices(IndexSubset *paramIndices) {
 
 GenericEnvironment *DifferentiableAttr::getDerivativeGenericEnvironment(
     AbstractFunctionDecl *original) const {
-  GenericEnvironment *derivativeGenEnv = original->getGenericEnvironment();
   if (auto derivativeGenSig = getDerivativeGenericSignature())
-    return derivativeGenEnv = derivativeGenSig->getGenericEnvironment();
+    return derivativeGenSig->getGenericEnvironment();
   return original->getGenericEnvironment();
 }