Remove Google+

Google+ is being shutdown on April 2, 2019. Social sharing buttong, comment provider, and author link configs have been removed from the theme.

ref: https://support.google.com/plus/answer/9195133

_sass/minimal-mistakes/_buttons.scss

@@ -39,7 +39,6 @@
   (info, $info-color),
   (facebook, $facebook-color),
   (twitter, $twitter-color),
-  (google-plus, $google-plus-color),
   (linkedin, $linkedin-color);
 
   @each $buttoncolor, $color in $buttoncolors {