[Libomptarget] Remove old workaround for GCC 5,6 from libomptarget

[jhuber6]

Summary

Some code previous needed the used attribute to prevent the GCC
compiler versions 5 and 6 from removing it. This is no longer required
as the minimum supported GCC version for LLVM 16 is >=7.1.0.

[JonChesterfield]

Like it, thanks!

[vzakhari]

LGTM. Thanks!

openmp/libomptarget/include/Debug.h

enum OpenMPInfoType : uint32_t {
   // Print kernel information from target device plugins.
   OMP_INFOTYPE_PLUGIN_KERNEL = 0x0010,
   // Print whenever data is transferred to the device
   OMP_INFOTYPE_DATA_TRANSFER = 0x0020,
   // Enable every flag.
   OMP_INFOTYPE_ALL = 0xffffffff,
 };
 
-#define GCC_VERSION                                                            \
-  (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
-
-#if !defined(__clang__) && defined(__GNUC__) && GCC_VERSION < 70100
-#define USED __attribute__((used))
-#else
-#define USED
-#endif
-
-// Add __attribute__((used)) to work around a bug in gcc 5/6.
-USED inline std::atomic<uint32_t> &getInfoLevelInternal() {
+inline std::atomic<uint32_t> &getInfoLevelInternal() {
   static std::atomic<uint32_t> InfoLevel;
   static std::once_flag Flag{};
   std::call_once(Flag, []() {
     if (char *EnvStr = getenv("LIBOMPTARGET_INFO"))
       InfoLevel.store(std::stoi(EnvStr));
   });
 
   return InfoLevel;
 }
 
-USED inline uint32_t getInfoLevel() { return getInfoLevelInternal().load(); }
+inline uint32_t getInfoLevel() { return getInfoLevelInternal().load(); }
 
-// Add __attribute__((used)) to work around a bug in gcc 5/6.
-USED inline uint32_t getDebugLevel() {
+inline uint32_t getDebugLevel() {
   static uint32_t DebugLevel = 0;
   static std::once_flag Flag{};
   std::call_once(Flag, []() {
     if (char *EnvStr = getenv("LIBOMPTARGET_DEBUG"))
       DebugLevel = std::stoi(EnvStr);
   });
 
   return DebugLevel;