Remove TestAssignmentManager#testGoodSplit, which no longer make sense

[ERROR] testGoodSplit(org.apache.hadoop.hbase.master.assignment.TestAssignmentManager) Time elapsed: 0.478 s <<< ERROR!
java.io.IOException: 5a50732f7cb3a05dd3a297bacbc34943 NOT splittable

GoodSplitExecutor can only mock some functions in RSProcedureDispatcher, however, to test split, we need to create(or mock) a real table on region server side. And GoodSplitExecutor can not mock those kinds of function.

And, similar test has already been covered in TestSplitTableRegionProcedure, we no longer need to test it again in TestAssignmentManager