text: Title: Remove `APIKeyMissingError`
  Body: Remove `APIKeyMissingError`. We are not using it anymore, we are using `MissingMandatoryKeyError` instead. 
  
  We only need to delete [it](https://github.com/scanapi/scanapi/blob/master/scanapi/errors.py#L15-L20)
  
  ```
  class APIKeyMissingError(MalformedSpecError):
      """Raised when 'api' key is not specified at root scope in the API spec"""
  
      def __init__(self, *args):
          message = "Missing api 'key' at root scope in the API spec"
          super(APIKeyMissingError, self).__init__(message, *args)
  ```