build: clear 227 warnings - dead private fields, malformed comments (#15376)

build: drop dead private fields, close malformed comments (227 warnings)

Clears 227 of the clang-cl warnings tracked in #15374, taking a full
Windows build from 1,491 to 1,264. Five of the six changes are in
headers, which are re-diagnosed in every translation unit that includes
them, so the count is large for a 14-line diff.

Tabbook.hpp: delete two private fields, unread since the 2022 import.
m_parent also shadowed wxWindowBase::m_parent.

GUI_Utils.hpp: the wxEVT_SYS_COLOUR_CHANGED lambda body is empty on
Windows, so its `this` capture is unused there. (void) this; leaves the
handler bound, which is what stops the event propagating.

DevFirmware.h: mark m_owner [[maybe_unused]]. The class is never
instantiated, and the file tracks BambuStudio, so this is the smallest
divergence.

Eight DeviceTab/ files, AMSItem.cpp and SelectMachine.cpp: block
comments malformed so that they read as a nested /*.

No behavior change. -Wcomment goes to zero, and only the three intended
categories move.

src/slic3r/GUI/SelectMachine.cpp

@@ -2847,7 +2847,7 @@ void SelectMachineDialog::on_ok_btn(wxCommandEvent &event)
             });
 
         // STUDIO-9580
-        /* use warning color if there are warning and normal messages* /
+        /* use warning color if there are warning and normal messages*/
         /* use indexes if there are several messages*/
         /* add header and ending if there are several messages or has none block warnings*/
         if (confirm_text.size() > 1 || !is_printing_block)