Update appReexports & remove unused util

[mkszepp]

There is not necessary to add appReexport for test-support & utils.

This brings warnings in embroider build process like:

WARNING in ../../../../ember-power-select/dist/_app_/utils/group-utils.js 1:0-63
export 'default' (reexported as 'default') was not found in 'ember-power-select/utils/group-utils' (possible exports: advanceSelectableOption, countOptions, defaultHighlighted, defaultMatcher, defaultTypeAheadMatcher, filterOptions, findOptionWithOffset, indexOfOption, isGroup, optionAtIndex, pathForOption, stripDiacritics)
 @ ./assets/test-app.js 37:13-53