refactor: remove duplicate tok.Token()

[jackwotherspoon]

In #909 we removed the need for refreshToken() but instead of removing the code we switched it to tok.Token()
which is already called so we can remove the duplicate.

cloud-sql-go-connector/internal/cloudsql/refresh.go

Lines 164 to 181 in dcd433d

	if tp != nil {
	var tokErr error
	tok, tokErr = tp.Token(ctx)
	if tokErr != nil {
	return tls.Certificate{}, errtype.NewRefreshError(
	"failed to retrieve Oauth2 token",
	inst.String(),
	tokErr,
	)
	}
	tok, tokErr = tp.Token(ctx)
	if tokErr != nil {
	return tls.Certificate{}, errtype.NewRefreshError(
	"failed to get Oauth2 token",
	inst.String(),
	tokErr,
	)
	}