Remove workaround for gcc 5.1

We support GCC 7 and up, so we can remove this.

absl/container/internal/raw_hash_set.h

@@ -1850,11 +1850,8 @@ class raw_hash_set {
   //   const char* p = "hello";
   //   const char* p = "hello";
   //   s.insert(p);
   //   s.insert(p);
   //
   //
-  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
-  // RequiresInsertable<T> with RequiresInsertable<const T&>.
-  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
   template <
   template <
-      class T, RequiresInsertable<T> = 0,
+      class T, RequiresInsertable<const T&> = 0,
       typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
       typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
   std::pair<iterator, bool> insert(const T& value) {
   std::pair<iterator, bool> insert(const T& value) {
     return emplace(value);
     return emplace(value);
@@ -1878,11 +1875,8 @@ class raw_hash_set {
     return insert(std::forward<T>(value)).first;
     return insert(std::forward<T>(value)).first;
   }
   }
 
 
-  // TODO(romanp): Once we stop supporting gcc 5.1 and below, replace
-  // RequiresInsertable<T> with RequiresInsertable<const T&>.
-  // We are hitting this bug: https://godbolt.org/g/1Vht4f.
   template <
   template <
-      class T, RequiresInsertable<T> = 0,
+      class T, RequiresInsertable<const T&> = 0,
       typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
       typename std::enable_if<IsDecomposable<const T&>::value, int>::type = 0>
   iterator insert(const_iterator, const T& value) {
   iterator insert(const_iterator, const T& value) {
     return insert(value).first;
     return insert(value).first;