Get rid of test_function_definition

[filipsch]

test_function_definition() is responsible for a lot of code in pythonwhat. If we get rid of this function, we can remove call() and test_function_definition() from the API. This would involve rewriting some SCTs though, for 39 exercises to be precise:

  course_id count link                                 
      <int> <int> <chr>                                
1      1392     6 https://www.datacamp.com/courses/1392
2      1531     2 https://www.datacamp.com/courses/1531
3      1532    15 https://www.datacamp.com/courses/1532
4      1549     4 https://www.datacamp.com/courses/1549
5      1550     4 https://www.datacamp.com/courses/1550
6      1650     3 https://www.datacamp.com/courses/1650
7      1822     5 https://www.datacamp.com/courses/1822

Not the most fun work, but it would make future maintenance and understanding of the package a lot easier.