The brackets are part of the BinaryOperatorExpression. Don't remove them.
Use the CodeAssignStatement instead of the CodeBinaryOperatorExpression.
inputmethod.Statements.Add(new CodeAssignStatement(
new CodeVariableReferenceExpression("ColA"),
new CodeArrayIndexerExpression(
new CodeVariableReferenceExpression("inputs"),
new CodePrimitiveExpression(0))));