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/libslic3r/GCode.cpp

@@ -9079,7 +9079,7 @@ bool GCode::needs_retraction(const Polyline &travel, ExtrusionRole role, LiftTyp
                     continue;
 
                 Polygons temp;
-                temp.emplace_back(std::move(instance_bbox.polygon()));
+                temp.emplace_back(instance_bbox.polygon());
                 if (intersection_pl(travel, temp).empty())
                     continue;