build: clear 50 warnings - pessimizing moves and null checks that cannot fail (#15408)

* build: remove std::move that blocks copy elision

std::move wrapped around a temporary, or around a local being returned,
stops the compiler constructing it in place. Each edit is the fix clang
suggests, which is to delete the std::move call and keep its argument.

Three of the 39 sites save a move, the two return std::move(local) in
Print.cpp and TreeSupport.cpp:2749. The rest are equivalent either way
and match how the codebase already writes this elsewhere.

Clears 39 -Wpessimizing-move warnings.

* build: drop null checks on references and this

A reference cannot be bound to null and this cannot be null, so the
compiler folds these conditions to true and drops the guard. Seven are
if (&bitmap && bitmap.IsOk()), where IsOk() already does the work; two
test this directly. The guarded code runs either way, so removing the
dead operand changes nothing.

Clears 11 -Wundefined-bool-conversion warnings.

src/slic3r/GUI/Plater.cpp

@@ -3870,7 +3870,7 @@ bool Sidebar::reset_bed_type_combox_choices(bool is_sidebar_init)
         }
     }
     m_last_combo_bedtype_count = p->combo_printer_bed->GetCount();
-    if (!is_sidebar_init && &p->plater->get_partplate_list()) {
+    if (!is_sidebar_init) {
         p->plater->get_partplate_list().check_all_plate_local_bed_type(m_cur_combox_bed_types);
     }
     return true;