Fix for showstopper defect MAGN-280

#### Background

This commit is meant to fix the following defect:

[MAGN-280 Dragging of selection area causes flickering of nodes](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-280)

From the way CrossSelectCommand, ContainSelectionCommand commands are implemented, there is no reason why selection needs to be cleared for each mouse-move. This is especially redundant since we are already clearing the selection at the beginning of window selection (during mouse-down).

#### Unit testing

All test cases passed: DynamoPythonTests, DynamoCoreUITests, DynamoCoreTests, DynamoMSOfficeTests and DSCoreNodesTests.