[TASK] Cleanup EXT:indexed_search

* Move modAccess check to constructor
* Move isLoaded check to constructor
* Remove check if extbase is loaded
* Replace $TYPO3_CONF_VARS to $GLOBALS['TYPO3_CONF_VARS']

Resolves: #56831
Releases: 6.2
Change-Id: Ib824d34219bc1afb646fbaf70b572b6bd3a55a47
Reviewed-on: https://review.typo3.org/28308
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind