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/GUI_Utils.hpp

@@ -155,6 +155,9 @@ public:
                 update_dark_config();
                 on_sys_color_changed();
                 event.Skip();
+#else
+                // Not calling Skip() is what stops the event propagating on Windows.
+                (void) this;
 #endif // __WINDOWS__
 
         });