Helper function to zap the common prefix of a string

[sfirke]

It goes with the implemented functions check_all_recode and check_all_count, for when the item text has a common prefix to be eliminated.

So if you have a lot of subquestions that start "Please indicate your level of agreement with the following statement" - all of those prefixes get zapped, with the text selected automatically (instead of having to gsub() it).