[clang-tidy][NFC] Remove another ad-hoc exclusion for system headers (#190094)

This is another ad-hoc approach made obsolete by #151035.

clang-tools-extra/clang-tidy/portability/StdAllocatorConstCheck.cpp

@@ -54,13 +54,7 @@ void StdAllocatorConstCheck::registerMatchers(MatchFinder *Finder) {
 
 void StdAllocatorConstCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *T = Result.Nodes.getNodeAs<TypeLoc>("type_loc");
-  if (!T)
-    return;
-  // Exclude TypeLoc matches in STL headers.
-  if (isSystem(Result.Context->getSourceManager().getFileCharacteristic(
-          T->getBeginLoc())))
-    return;
-
+  assert(T);
   diag(T->getBeginLoc(),
        "container using std::allocator<const T> is a deprecated libc++ "
        "extension; remove const for compatibility with other standard "