Sharing: make unneeded external HTTP call even when email is not enabled

#### Steps to reproduce the issue

1. Install Query Monitor plugin
2. Enable share module without email service
3. Go to a post page
4. When the time is very long to load the page like:
5. Hover the area in the image above & there should be something like "HTTP API Calls (1)", click on it
6. You will see the one in the screenshot section below

#### What I expected

No HTTP call should be made on the server side

#### What happened instead

While getting a list of all services, it made a call to akismet by `Jetpack::is_akismet_active`:
https://github.com/Automattic/jetpack/blob/master/modules/sharedaddy/sharing-service.php#L83-L85

if ( apply_filters( 'sharing_services_email', Jetpack::is_akismet_active() ) ) {
$services['email'] = 'Share_Email';
}

**Screenshots**
If applicable, add screenshots to help explain your problem.

The email service is not enabled, but a HTTP call to akismet is still made:


The relevant logic is in modules/sharedaddy/sharing-service.php at lines 83-85, where the sharing services are assembled. Reproduce the issue with Query Monitor while the email service is disabled, then verify that loading a post page makes no HTTP call to Akismet when email sharing is unavailable.

Written by the indexing model from the issue text.