Work around an MSVC 2013 bug related to unique_ptr.

The following bug is tickled by calling emplace() on a map with
non-copyable but moveable values:
https://connect.microsoft.com/VisualStudio/feedbackdetail/view/960021

Fortunately, we don't actually need emplace() here as operator[]
will create values if they aren't there.

src/importdxf.cpp

@@ -537,7 +537,6 @@ public:
     }
 
     virtual void addBlock(const DRW_Block &data) {
-        blocks.emplace(data.name, Block());
         readBlock = &blocks[data.name];
         readBlock->data = data;
     }