Symmetric slicers not getting removed when plot is deleted

[jellybean2004]

Describe the bug

When a symmetric slicer plot gets deleted, the relevant slicer do not get removed. When te slicers are updated after plot deletion, warnings are logged.`

As per #3970, the slicers should get removed when the plot is deleted.

09:50:22 - WARNING: Failed to post data for master slicer: libshiboken: Internal C++ object (PlotterWidget) already deleted.
09:50:22 - WARNING: Failed to post data for slicer 2: libshiboken: Internal C++ object (PlotterWidget) already deleted.
09:50:22 - WARNING: Failed to post data for slicer 3: libshiboken: Internal C++ object (PlotterWidget) already deleted

#3970
To Reproduce
Steps to reproduce the behavior:

1. Plot a 2D data
2. Add symmetric slicers
3. Delete the slicer generated plot
4. Move the slicer and see warning

Expected behavior
Slicers should get removed if their plots are deleted.

SasView version:

• Version: 6.2.0b1 and main

Operating system:

• OS: Windows 10

Installation method:

• Installed: Windows Installer and pip