chore(web): Investigate possible removal of `adjustDeadkeys`

It looks like we may be able to remove adjustDeadkeys altogether, as I believe it relates to obsolete IE code. Needs careful review.

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> Admittedly, I've always been a bit fuzzy on exactly what sort of case this was meant to address.

It's a long time ago, but I *think* it relates to IE's text selection support. I have a feeling it may be completely obsolete now. IE's selection management did not give an offset relative to start of text, so we had everything calculated as relative offsets to the 'selection' (caret if selection is empty). Hence, any changes to context required a recalculation of our cached deadkey offsets. However, once we have deadkey offsets in absolute positions, then deletion of content just requires deletion of deadkeys that fall to the right of the caret -- and that should already be working, of course.

We should do some careful testing, I guess, to see if this feeling of mine is correct, but if so, time for a little happy dance: simplifying complex logic is a Good Thing.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/7345#issuecomment-1258999974_