[SYCL] Simplify check whether a header file is available for inclusion (#19759)

`__has_include` is part of the C++ standard since C++17:
https://en.cppreference.com/w/cpp/preprocessor/include

We require C++17 if `sycl.hpp` is included, so we can use
`__has_include` without extra checks for its presence.

Author: dyniols

sycl/include/sycl/khr/group_interface.hpp

@@ -13,11 +13,9 @@
 
 #define SYCL_KHR_GROUP_INTERFACE 1
 
-#if __cplusplus >= 202302L && defined(__has_include)
-#if __has_include(<mdspan>)
+#if __cplusplus >= 202302L && __has_include(<mdspan>)
 #include <mdspan>
 #endif
-#endif
 
 namespace sycl {
 inline namespace _V1 {

sycl/source/detail/os_util.cpp

@@ -13,10 +13,10 @@
 #include <limits>
 
 // For GCC versions less than 8, use experimental/filesystem.
-#if defined(__has_include) && __has_include(<filesystem>)
+#if __has_include(<filesystem>)
 #include <filesystem>
 namespace fs = std::filesystem;
-#elif defined(__has_include) && __has_include(<experimental/filesystem>)
+#elif __has_include(<experimental/filesystem>)
 #include <experimental/filesystem>
 namespace fs = std::experimental::filesystem;
 #else