[BUGFIX] Move forgotten getEnvironmentMode()

getWhereClauseForEnabledFields() moved from StorageRepository
to AbstractRepository, but getEnvironmentMode() stayed
in StorageRepository although getWhereClauseForEnabledFields()
relies on it. Move getEnvironmentMode() as well.

Change-Id: Id252c8bd3b9e09a2c38d5ea1ebe6497dd76c12ae
Resolves: #51562
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/23759
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel