MDL-56979 group: Template and styles fixes

* Remove custom styles for groups since we're using default Bootstrap
  styles now.
* Hidden input breaks grid layout. Moved it inside the groups column.
* Removed unnecessary 'groups' and 'members' classes. They are not being
  used in JS nor CSS.

theme/boost/scss/moodle/core.scss

@@ -346,9 +346,6 @@ a.skip:active {
     margin-left: 43px;
 }
 // Group
-#page-group-index #groupeditform {
-    text-align: center;
-}
 
 #doc-contents h1 {
     margin: 1em 0 0 0;

theme/boost/scss/moodle/user.scss

@@ -277,21 +277,6 @@
     }
 }
 
-#groupeditform {
-    .groups,
-    .members {
-        min-width: 175px;
-        width: 49%;
-        float: left;
-        text-align: left;
-
-        select {
-            min-width: 175px;
-            max-width: 90%;
-        }
-    }
-}
-
 // Remove the little cog from participants page because we are putting a cog menu there.
 .userlist h3 .action-icon {
     display: none;