fix: remove export_test.go which is unused

The two variables exposed in export_test aren't used anywhere anymore,
so it should be safe to remove export_test altogether.

export_test.go

@@ -1,13 +0,0 @@
-// Expose some internals for testing purposes
-package lz4
-
-// expose the possible block max sizes
-var BlockMaxSizeItems []int
-
-func init() {
-	for s := range bsMapValue {
-		BlockMaxSizeItems = append(BlockMaxSizeItems, s)
-	}
-}
-
-var FrameSkipMagic = frameSkipMagic