[DF] Simplify RTreeColumnReader destructor

Since commit 3b623b2 ("[DF] Use dtors to perform clean-up tasks in
column readers"), the destructor resets the std::unique_ptr holding a
TTreaderValue, which is the default of the pointer's destructor.

tree/dataframe/inc/ROOT/RDF/RTreeColumnReader.hxx

@@ -37,16 +37,6 @@ public:
  : fTreeValue(std::make_unique<TTreeReaderValue<T>>(r, colName.c_str()))
  {
  }
-
- /// The dtor resets the TTreeReaderValue object.
- //
- // Otherwise a race condition is present in which a TTreeReader
- // and its TTreeReader{Value,Array}s can be deleted concurrently:
- // - Thread #1) a task ends and pushes back processing slot
- // - Thread #2) a task starts and overwrites thread-local TTreeReaderValues
- // - Thread #1) first task deletes TTreeReader
- // See https://github.com/root-project/root/commit/26e8ace6e47de6794ac9ec770c3bbff9b7f2e945
- ~RTreeColumnReader() override { fTreeValue.reset(); }
 };
 
 /// RTreeColumnReader specialization for TTree values read via TTreeReaderArrays.
@@ -135,9 +125,6 @@ public:
  : fTreeArray(std::make_unique<TTreeReaderArray<T>>(r, colName.c_str()))
  {
  }
-
- /// See the other class template specializations for an explanation.
- ~RTreeColumnReader() override { fTreeArray.reset(); }
 };
 
 /// RTreeColumnReader specialization for arrays of boolean values read via TTreeReaderArrays.
@@ -176,9 +163,6 @@ public:
  : fTreeArray(std::make_unique<TTreeReaderArray<bool>>(r, colName.c_str()))
  {
  }
-
- /// See the other class template specializations for an explanation.
- ~RTreeColumnReader() override { fTreeArray.reset(); }
 };
 
 } // namespace RDF