GUI App : Remove unnecessary `del`.
GUI App : Remove unnecessary `del`.

This should have been removed in 45a0ff5526deb1f6ecddefc93bc97f3fbd0f5d33. Prior to that commit, the `_run()` method performed a check that all ScriptNodes had been destroyed before shutdown, and it was therefore necessary to explicitly drop the reference held by the local `scriptNode` variable. Following that commit, the checks are performed after `_run()` returns, after the `scriptNode` variable has left scope anyway.

ImageEngine/gaffer,boberfly/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,boberfly/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,boberfly/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,boberfly/gaffer,lucienfostier/gaffer,hradec/gaffer,lucienfostier/gaffer