[thrax] Drop unnecessary std::less<> from maps used internally

* These were added a while back in order to let `std::string` STL containers deal with `absl::string_view` keys without copying
* Now that we are using Abseil containers, this is dealt with for us

PiperOrigin-RevId: 930819014

Author: lwolfsonkin

opengrm/thrax/abstract-grm-manager.h

@@ -50,8 +50,8 @@ class AbstractGrmManager {
  public:
   using Transducer = ::fst::Fst<Arc>;
   using MutableTransducer = ::fst::VectorFst<Arc>;
-  using FstMap = absl::btree_map<std::string, std::unique_ptr<const Transducer>,
-                                 std::less<>>;
+  using FstMap =
+      absl::btree_map<std::string, std::unique_ptr<const Transducer>>;
   using Label = typename Arc::Label;
 
   virtual ~AbstractGrmManager();

opengrm/thrax/walker/evaluator.h

@@ -390,9 +390,9 @@ class AstEvaluator : public AstWalker {
   // grammar). This information gets passed down ultimately to StringFst's
   // GetLabelSymbolTable to determine (assuming --save_symbols is set), whether
   // or not to add generated labels to the byte and utf8 symbol tables.
-  void GetFsts(absl::btree_map<std::string, std::unique_ptr<const Transducer>,
-                               std::less<>>* fsts,
-               bool top_level) {
+  void GetFsts(
+      absl::btree_map<std::string, std::unique_ptr<const Transducer>>* fsts,
+      bool top_level) {
     // Checks if we ever used generated labels. If so, get the symbol table and
     // add it to a unique FST called kStringFstSymtabFst.
     if (auto generated_labels =