removed unused DataBLockId (#405)

variable is not referenced. 
will allow cleanup of Common/DataBLock.h

Author: sy-c

Modules/Daq/include/Daq/EverIncreasingGraph.h

@@ -40,8 +40,6 @@ class EverIncreasingGraph : public o2::quality_control::checker::CheckInterface
   std::string getAcceptedType() override;
 
  private:
-  DataBlockId mLastId;
-
   ClassDefOverride(EverIncreasingGraph, 1);
 };