Removing a character from a string
Notice that the loop will only occur once! In the beginning num will be 0. At this point, we enter the if statement:

if we got that str[0] == ch (notice that num==0 and thus str[0]==str[num]), then we return some string
otherwise we return another string

The important thing here, is that no matter what - we will quit in the first iteration of the for loop.
To fix this, consider placing the return str outside the for loop (without else as well)