NFC: Remove unused variable in Deserialization.cpp.

Author: tshortli

lib/Serialization/Deserialization.cpp

@@ -7625,7 +7625,6 @@ class SwiftToClangBasicReader :
     if (!rawKind)
       return nullptr;
 
-    clang::attr::Kind attrKind = static_cast<clang::attr::Kind>(rawKind - 1);
     auto name = readIdentifier();
     auto scopeName = readIdentifier();