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/bucket.go

@@ -288,7 +288,6 @@ func (b bucket) WriteObject(objectName string, objectData io.Reader, size int64,
 		objMetadata.ChunkCount = chunkCount
 		objMetadata.DataDisks = k
 		objMetadata.ParityDisks = m
-		objMetadata.ErasureTechnique = "Cauchy"
 		objMetadata.Size = int64(totalLength)
 	}
 	objMetadata.Bucket = b.getBucketName()
@@ -431,7 +430,7 @@ func (b bucket) getDataAndParity(totalWriters int) (k uint8, m uint8, err *probe
 
 // writeObjectData -
 func (b bucket) writeObjectData(k, m uint8, writers []io.WriteCloser, objectData io.Reader, size int64, hashWriter io.Writer) (int, int, *probe.Error) {
-	encoder, err := newEncoder(k, m, "Cauchy")
+	encoder, err := newEncoder(k, m)
 	if err != nil {
 		return 0, 0, err.Trace()
 	}
@@ -503,11 +502,7 @@ func (b bucket) readObjectData(objectName string, writer *io.PipeWriter, objMeta
 	mwriter := io.MultiWriter(writer, hasher, sum512hasher)
 	switch len(readers) > 1 {
 	case true:
-		if objMetadata.ErasureTechnique == "" {
-			writer.CloseWithError(probe.WrapError(probe.NewError(MissingErasureTechnique{})))
-			return
-		}
-		encoder, err := newEncoder(objMetadata.DataDisks, objMetadata.ParityDisks, objMetadata.ErasureTechnique)
+		encoder, err := newEncoder(objMetadata.DataDisks, objMetadata.ParityDisks)
 		if err != nil {
 			writer.CloseWithError(probe.WrapError(err))
 			return