HelpWanted: Remove last character then find index of sep string

I do some string parse work.
Remove last character then find index of sep string.

I really want is: Does there have a easy way(less code and better performance) to resolve my problem?
If don't, How can I compare the three way I wrote(both memory used and performance)?

There are what method I found:

https://play.golang.org/p/2H84WYu2Mp
https://play.golang.org/p/JyFWZTtTfe

https://play.golang.org/p/VpHzAZXk6b

Thanks for your time!

Small question: I found there are many method for help []byte and string in strings, bytes.
Why there don't have same methods help []rune?

Update:
Remove this. I think about []int []int64 []float64 []int32 []bool and etc.

Close this. Found blog talk about this.
https://blog.golang.org/strings