AMP: remove deprecated Google+ icon (#12024)

#### Changes proposed in this Pull Request:

Google+ is no more. Remove button from AMP compat class.

#### Testing instructions:

* Not much since the button is now gone.

#### Proposed changelog entry for your changes:

* None

3rd-party/class.jetpack-amp-support.php

@@ -265,9 +265,6 @@ static function render_sharing_html( $markup, $sharing_enabled ) {
 			'twitter'       => array(),
 			'pinterest'     => array(),
 			'whatsapp'      => array(),
-			'google-plus-1' => array(
-				'type' => 'gplus',
-			),
 			'tumblr'        => array(),
 			'linkedin'      => array(),
 		);