video_core/control_flow: Remove unnecessary BlockStack copy constructor

This is the default behavior of the copy constructor, so it doesn't need
to be specified.

While we're at it we can make the other non-default constructor
explicit.

src/video_core/shader/control_flow.cpp

@@ -29,8 +29,7 @@ struct Query {
 
 struct BlockStack {
     BlockStack() = default;
-    BlockStack(const BlockStack& b) = default;
-    BlockStack(const Query& q) : ssy_stack{q.ssy_stack}, pbk_stack{q.pbk_stack} {}
+    explicit BlockStack(const Query& q) : ssy_stack{q.ssy_stack}, pbk_stack{q.pbk_stack} {}
     std::stack<u32> ssy_stack{};
     std::stack<u32> pbk_stack{};
 };