Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• PartsBinPaletteWidget
• ItemDrag::cleanup
• SvgIconWidget::cleanup

[KjellMorgenstern]

Indeed, it is safe to call delete null_ptr .
Also PartsBinPalette might set m_icon to nullptr after deleting.
Are you working on a PR?

[elfring]

How do you think about to use a development tool like “clang-tidy” for corresponding source code adjustments?

[KjellMorgenstern]

Of course, use it. I do. But still, fixes should not break anything. This is problematic without automated tests: Reviewing a fix for a harmless warning takes often more time than fixing an actual real bug that had been reported on github.