input: Write a python function to remove all occurrences of a specified letter from a string.
  assert remove_occurences('a', 'banana') == 'bnn'
  assert remove_occurences('e', 'elephant') == 'lphant'
  assert remove_occurences('x', 'example') == 'example'
output: def remove_occurences(letter, string):
      return string.replace(letter, '')