fix: remove duplicate layout parent in SidePanelWidget (#4578)

The widget's layout is set in the member initializer list (line 25)
via m_layout(new QVBoxLayout(this)). Passing 'this' to the QHBoxLayout
constructor at line 84 tries to set a second layout on the same widget,
which Qt rejects with a warning on every capture:

  QLayout: Attempting to add QLayout "" to SidePanelWidget "",
  which already has a layout

The parent argument is unnecessary since the layout is added to
m_layout via addLayout() at line 94.

Co-authored-by: Benigno Batista Júnior <[email protected]>