Remove useless and dead code.

* A function which splitted a line into words.
* A function which checked if the line was a commented line (a function
  'is_comment' carries out this checking)
* The related test cases.
[str_utils] Rename the functions which checks a valid char for a float.
[str_utils.test] Missing a unit test for 'first_char_is_not_valid_float'.