chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/openvpn/extract/extractor.go

@@ -1,16 +1,5 @@
 package extract
 
-import (
-	"github.com/qdm12/gluetun/internal/models"
-)
-
-var _ Interface = (*Extractor)(nil)
-
-type Interface interface {
-	Data(filepath string) (lines []string,
-		connection models.Connection, err error)
-}
-
 type Extractor struct{}
 
 func New() *Extractor {