Revert "Update restyle.less (#4266)"

This reverts commit 6cbc938a0f.

We ran into a regression with various additions to the .addon-header.
Let's fix this in a different way.

static/css/restyle/restyle.less

@@ -263,7 +263,6 @@ button.link:active span {
 
 .amo-header {
   margin-bottom: 0;
-  padding-bottom: 15px;
 }
 
 .amo-header-wrapper {