Setting isCellEntered to true already makes that same call, so *should* have that same effect as long as we're consistent about changing logic via isCellEntered

This also fixes elastic#4384, which was a focus-lock issue caused focusin/focusout events firing multiple times. Now with the setIsCellEntered change, focus does not fire repeatedly

- now that we're not calling enableInteractives and focusInteractives directly but letting isCellEntered handle that for us, there's no need to include them

+ setIsCellEntered isn't necessary either/doesn't trigger the lint rule

- Separate focus tests for isFocused logic vs focusin / focusout events

- Removes the need for useCallback and setting the utility fns as dependencies, simplifying code

- refactor for loops to forEach's

- remove setIsCellEntered(true) in focusInteractables, since it now only gets called when isCellEntered is true

- they don't get called separately, so it makes sense to optimize the call and not make an extra tabbables call (+ reduces unnecessary branching)

- covers last uncovered branch in file

- move to bottom of the file rather than top, since after talking to Chandler this is an edge case that only applies to control header cells

+ remove `data-euigrid-tab-managed` attr - tests should pass without it

- convert functions to arrow functions
- use shorter headerNode var instead of ref

- it's had no effect since we switched header cells to use a popover for actions