src: simplify is_callable by making it a concept

src: simplify is_callable by making it a concept · nodejs/node@69b5607

Original file line number	Diff line number	Diff line change
`@@ -661,13 +661,9 @@ struct MallocedBuffer {`
`661`	`661`	`};`
`662`	`662`
`663`	`663`	`// Test whether some value can be called with ().`
`664`		`-template <typename T, typename = void>`
`665`		`-struct is_callable : std::is_function<T> { };`
`666`		`-`
`667`	`664`	`template <typename T>`
`668`		`-struct is_callable<T, typename std::enable_if<`
`669`		`- std::is_same<decltype(void(&T::operator())), void>::value`
`670`		`->::type> : std::true_type { };`
	`665`	`+concept is_callable =`
	`666`	`+ std::is_function<T>::value \|\| requires { &T::operator(); };`
`671`	`667`
`672`	`668`	`template <typename T, void (function)(T)>`
`673`	`669`	`struct FunctionDeleter {`