A string is a group of characters in a specified order, called sequence of characters.
This sequences are zero-indexed.
To remove a character from a string in TSQL use SUBSTRING.
The substring is a built-in function that takes, expression, starting point and length of the expression.
Based on those three parameters, it decides on what string to return
To remove the last character, what do you have to do?
Think about it, all you need to do is change the starting point value from 2 to 1.