[clang] Remove unused variable [NFC]

Without the fix gcc warned like
 ../../clang/lib/Analysis/UnsafeBufferUsage.cpp:2203:26: warning: unused variable 'CArrTy' [-Wunused-variable]
  2203 |   } else if (const auto *CArrTy = Ctx.getAsConstantArrayType(
       |                          ^~~~~~