Remove replace directives and use own fork to make go get work again (#1028)

See https://github.com/golang/go/issues/30354
go get doesn't honor the go.mod replace options.

vendor/github.com/yaegashi/msgraph.go/msauth/client_credentials_grant.go

@@ -1,27 +0,0 @@
-package msauth
-
-import (
-	"context"
-
-	"golang.org/x/oauth2"
-	"golang.org/x/oauth2/clientcredentials"
-	"golang.org/x/oauth2/microsoft"
-)
-
-// ClientCredentialsGrant performs OAuth 2.0 client credentials grant and returns auto-refreshing TokenSource
-func (m *Manager) ClientCredentialsGrant(ctx context.Context, tenantID, clientID, clientSecret string, scopes []string) (oauth2.TokenSource, error) {
-	config := &clientcredentials.Config{
-		ClientID:     clientID,
-		ClientSecret: clientSecret,
-		TokenURL:     microsoft.AzureADEndpoint(tenantID).TokenURL,
-		Scopes:       scopes,
-		AuthStyle:    oauth2.AuthStyleInParams,
-	}
-	var err error
-	ts := config.TokenSource(ctx)
-	_, err = ts.Token()
-	if err != nil {
-		return nil, err
-	}
-	return ts, nil
-}