[Indexers] Remove no-op recursion over includedFolders, remove incorrect mimetype check

For any of the includeFolders shouldDelete will always return false, and the recursion will be stopped immediately, so the loop is pointless.

• QFile::exist(url) is implied by filePathToId > 0
• shouldBeIndexed(url) is always true for includeFolders
• the mimetype check is pointless for directories

Drop the mimetype check in xattrindexer and indexcleaner, as done everywhere else.