Fix xt::drop with variables

[Alex-PLACET]

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Description

Address #2867

Changed xtl::is_integral<T>::value to xtl::is_integral<std::decay_t<T>>::value in the drop() function template. When passing a variable like size_t index = 1, T deduces to unsigned long&, a reference type, which is_integral rejects. Decaying strips the reference so the integral branch is correctly taken.