Revert "Added index method to InstructionGroup"

This reverts commit cc9af2bb191338f31225c192df9739a28e29f197.

This commit only duplicates the functionality of
InstructionGroup.indexof.
Merge pull request #1888 from inclement/master

Revert "Added index method to InstructionGroup"

I agree index is a better name, and a better implementation is probably worthwhile, but I think a method rename may as well wait until version 2 with the other breaking changes that have accrued. I guess improving the existing method can go in a different pr, I will merge this one for now.