Remove unneeded guard condition after GAPIC generator rollout of WithDefaultEndpointTemplate

[quartzmo]

Once the GAPIC generator rollout of WithDefaultEndpointTemplate is complete, remove the following guard condition from around internal/cba.go#L79:

&& settings.DefaultEndpointTemplate != ""

And uncomment the error check just below:

// if settings.DefaultEndpointTemplate == "" {
// 	return nil, "", errors.New("internaloption.WithDefaultEndpointTemplate is required if option.WithUniverseDomain is not googleapis.com")
// }

[codyoss]

@quartzmo can this be acted on?

[quartzmo]

@codyoss I removed every usage of DefaultEndpointTemplate that I could find. So now is as good as time as any. Thanks for the ping, I'll create a PR.

[codyoss]

I believe this has been done for a bit now.