Make erasure matrix type not optional choose automatically

Remove option of providing Technique and handling errors based on that
choose a matrix type automatically based on number of data blocks.

INTEL recommends on using cauchy for consistent invertible matrices,
while vandermonde is faster we should default to cauchy for large
data blocks.

pkg/donut/definitions.go

@@ -30,11 +30,10 @@ type ObjectMetadata struct {
 	Size    int64     `json:"size"`
 
 	// erasure
-	DataDisks        uint8  `json:"sys.erasureK"`
-	ParityDisks      uint8  `json:"sys.erasureM"`
-	ErasureTechnique string `json:"sys.erasureTechnique"`
-	BlockSize        int    `json:"sys.blockSize"`
-	ChunkCount       int    `json:"sys.chunkCount"`
+	DataDisks   uint8 `json:"sys.erasureK"`
+	ParityDisks uint8 `json:"sys.erasureM"`
+	BlockSize   int   `json:"sys.blockSize"`
+	ChunkCount  int   `json:"sys.chunkCount"`
 
 	// checksums
 	MD5Sum    string `json:"sys.md5sum"`