Do not require GITHUB_TOKEN for enterprise setups

Not sure why this was added but using no token works for our GitHub enterprise installation.

selfupdate/updater.go

@@ -2,7 +2,6 @@ package selfupdate
 
 import (
 	"context"
-	"errors"
 	"net/http"
 	"os"
 
@@ -56,9 +55,6 @@ func NewUpdater(config Config) (*Updater, error) {
 		return &Updater{client, ctx}, nil
 	}
 
-	if token == "" {
-		return nil, errors.New("GitHub API token cannot be empty when releases are hosted on GitHub Enterprise instance")
-	}
 	u := config.EnterpriseUploadURL
 	if u == "" {
 		u = config.EnterpriseBaseURL