Remove Set self support from pgtype

Set having the capability to assign an object of the same type was
inconsistently implemented. Some places it was not implemented at all, some
places it was a shallow copy, some places a deep copy. Given that it doesn't
seem likely to ever be used, and if it is needed it is easy enough to do
outside of the library this code has been removed.

float8_array.go

@@ -17,8 +17,6 @@ type Float8Array struct {
 
 func (dst *Float8Array) Set(src interface{}) error {
 	switch value := src.(type) {
-	case Float8Array:
-		*dst = value
 
 	case []float64:
 		if value == nil {