Dead code in trunk

I did some code analysis, and as a byproduct was able to identify some dead code in the form of files that can be safely removed. After filtering out some false positives, this is what remained:

gms/IFailureNotification.java

• was there since FB open-sourced Cassandra; has never been used for anything. No classes implement the interface, and it's not mentioned anywhere in the codebase.

service/PendingRangeCalculatorServiceMBean.java

• empty MBean used as a base class for PendingRangeCalculatorService, but has not been touched since being introduced several months ago. NOTE: removing this will require editing PendingRangeCalculatorService to not derive from this anymore.

db/ColumnFamilyNotDefinedException.java

• used to be thrown in original FB Cassandra; no longer used anywhere.

db/context/IContext.java

• introduced in 2c4ac98c9ffa8ea52da801830c7cdb745ddc28f0 (CASSANDRA-1072); was used extensively then, but no longer used anywhere.

db/columniterator/SimpleAbstractColumnIterator.java

• introduced in 48093358fb9022947592813a6aae43db148847ca (CASSANDRA-287); was used then; no longer used anywhere.

thrift/RequestType.java

• enum introduced in 72199e23ec9d604449bef87733a32e1da9924437 (CASSANDRA-3272); was used then; no longer used anywhere.

utils/AtomicLongArrayUpdater.java

• introduced in 22e18f5a348a911f89deed9f9984950de451d28a (CASSANDRA-3578), but has never been used for anything. Not sure what the original intent might have been.

utils/DefaultDouble.java

• introduced in 96588d4f322dfbb1f5ff9328afe4377babfb1d2c (CASSANDRA-1715); was used then; no longer used anywhere.

utils/LatencyTracker.java

• introduced in 979a022f896aaa5a799b27a973cd476e5727820e (CASSANDRA-702); was used then; no longer used anywhere.

utils/SkipNullRepresenter.java

• introduced in a6777492280ae481392cd4cb4ba613923f84989d(CASSANDRA-1133) was used then; no longer used anywhere.