Remove nonsensical operator+= from Gaudi::Accumulators::Counter

The basic counter Gaudi::Accumulators::Counter should only be incremented via pre/postfix ++, but it still inherits operator+= from the base class which increments the counter by one despite the value passed.

So the lines

m_counter++;
m_counter += 1234;

have exactly the same effect, but the second one is very confusing so it should be prohibited.