Revert D6851455: [folly][PR] Add missing override for adjacent_tokens_only

Summary:
This reverts commit 5ad448f0aad8f51f4328bfe9af37d63023089e88

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Differential Revision: D6851455

fbshipit-source-id: d96ac9c2f907f09a4086ccf65ec4a3ef47cad205

folly/experimental/ProgramOptions.cpp

@@ -83,7 +83,7 @@ class GFlagValueSemanticBase : public po::value_semantic {
 
   std::string name() const override { return "arg"; }
 #if BOOST_VERSION >= 105900
-  bool adjacent_tokens_only() const override {
+  bool adjacent_tokens_only() const {
     return false;
   }
 #endif