[clang] Remove unused variable 'ExprDependenceBits' in ASTWriterDecl.cpp (NFC)

llvm-project/clang/lib/Serialization/ASTWriterDecl.cpp:2342:12:
 error: unused variable 'ExprDependenceBits' [-Werror,-Wunused-variable]
 2342 |   unsigned ExprDependenceBits = llvm::BitWidth<ExprDependence>;
      |            ^~~~~~~~~~~~~~~~~~
1 error generated.