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/errors.go

@@ -132,22 +132,6 @@ func (e MissingPOSTPolicy) Error() string {
 	return "Missing POST policy in multipart form"
 }
 
-// MissingErasureTechnique missing erasure technique
-type MissingErasureTechnique struct{}
-
-func (e MissingErasureTechnique) Error() string {
-	return "Missing erasure technique"
-}
-
-// InvalidErasureTechnique invalid erasure technique
-type InvalidErasureTechnique struct {
-	Technique string
-}
-
-func (e InvalidErasureTechnique) Error() string {
-	return "Invalid erasure technique: " + e.Technique
-}
-
 // InternalError - generic internal error
 type InternalError struct {
 }