Remove eval_op_concat

eval_op_concat has code to search for an operator overload of
BINOP_CONCAT.  However, the operator overloading code is specific to
C++, which does not have this operator.  And,
binop_types_user_defined_p rejects this case right at the start, and
value_x_binop does not handle this case.  I think this code has been
dead for a very long time.  This patch removes it and hoists the
remaining call into concatenation::evaluate, removing eval_op_concat
entirely.