Fix ABSC reverse iterator's remove() method

patch by Mikhail Stepura; reviewed by Aleksey Yeschenko for
CASSANDRA-6629
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/ArrayBackedSortedColumns.java