Merged calculatePrimes() with calculateOddPrimes()

Given that the calculatePrimes() method was initially only 2 lines of code and a comment, it was safe to assume that the logic it held could be incorporated into calculateOddPrimes. 
Therefore, 
- calculatePrimes() method's content was copied into calculateOddPrimes
- calculatePrimes() was deleted
- calculateOddPrimes was renamed to calculatePrimes() 
- the comment was changed to reflect the new, simplified logic of why the  number 2 is added to the list manually
- the index at which the number 2 is added to the list was changed from 1 to 0. Otherwise, the position 0 of the list listOfPrimes would hold an empty or garbage value and thus it would give an erroneous calculation.