DiscIO: Move scrubbing code out of ConvertToGCZ

This way, scrubbing can also be performed when converting
to other formats.

Source/Core/DiscIO/DiscScrubber.cpp

@@ -25,8 +25,6 @@
 
 namespace DiscIO
 {
-constexpr size_t CLUSTER_SIZE = 0x8000;
-
 DiscScrubber::DiscScrubber() = default;
 DiscScrubber::~DiscScrubber() = default;