(CR-6590) Remove code specific to extensible indexes in Importer and VerifyJob 

AttributeIndex.java:
Removed extensibleIndexesMapping field + getter getExtensibleIndexes() + computeExtensibleIndexesMapping() and isDefaultIndex().
Removed getDefaultNameToIndexes(), superseded by getNameToIndexes()

Importer.java:
Removed extensibleIndexMap, superseded by indexMap.
Removed one fillIndexMap(), superseded by the other fillIndexMap()
Removed processExtensibleIndexes().

VerifyJob.java:
Consequence of removing AttributeIndex.getDefaultNameToIndexes().