Minor cleanup of Least

Remove custom method in Signatures to create a specialized signature for least.
That's not necessary outside of the Least class and it's an implementation detail.
Update to airbase 29

This makes the dependency plugin compatible with Java 8
Remove unused methods
Remove getter functions from ByteCodeExpression
Remove getter functions from JmxColumnHandle
Remove getter functions from JmxRecordSetProvider
Remove getter functions from QueryInfo
Remove getter functions from SqlStageExecution
Remove getter functions from SqlTask
Remove getter functions from StageInfo
Remove getter functions from TaskInfo
Remove getter/predicate functions from HeartbeatFailureDetector
Remove getter functions from ColumnHandle
Remove getter functions from FunctionInfo
Remove getter functions from MetadataUtil
Remove getter functions from ParametricFunctionUtils
Remove getter functions from Partition
Remove getter functions from OperatorStats
Remove getter functions from PageBufferClientStatus
Remove getter functions from PipelineContext