Debugger: Remove LLDB's Dumper.defaultValue

This was needed with earlier, now unsupported versions of LLDB to
provide a target context for evaluation of expressions.

Not needed anymore since SBTarget.EvaluateExpression() exists now.

Change-Id: I31871937832439dbd98b763ac77eab4db56ef2ee
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>