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/Printer/PrinterFileSystem.cpp

@@ -662,10 +662,10 @@ PrinterFileSystem::File const &PrinterFileSystem::GetFile(size_t index, bool &se
 void PrinterFileSystem::Attached()
 {
     boost::unique_lock lock(m_mutex);
-    m_recv_thread = std::move(boost::thread([w = weak_from_this()] {
+    m_recv_thread = boost::thread([w = weak_from_this()] {
         boost::shared_ptr<PrinterFileSystem> s = w.lock();
         if (s) s->RecvMessageThread();
-    }));
+    });
 }
 
 void PrinterFileSystem::Start()