Write a function remove$\_$characters$($input$\_$string, to$\_$remove$)$ that takes two inputs:

$$ input$\_$string : a string $$ to$\_$remove: a string

and returns a string that is exactly the same as the input$\_$string but without any of the characters in to$\_$remove. Your function must have a running time complexity of O$($n$),$ where n represents the length of input$\_$string.

For clarity, let m be the length of to$\_$remove. Your code cannot have a running time complexity of O$($mn$)$ to receive full credit, it must be O$($n$).$