Revert making the markdown fullscreen button tab-accessible

When fullscreen is activated via the markdown textarea
fullscreen button, the user cannot exit fullscreen
using only the keyboard

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -186,6 +186,7 @@ export default {
             :aria-label="__('Go full screen')"
             class="toolbar-btn toolbar-fullscreen-btn js-zen-enter"
             data-container="body"
+            tabindex="-1"
             :title="__('Go full screen')"
             type="button"
           >

app/views/shared/blob/_markdown_buttons.html.haml

@@ -9,5 +9,5 @@
   = markdown_toolbar_button({ icon: "list-task", data: { "md-tag" => "- [ ] ", "md-prepend" => true }, title: _("Add a task list") })
   = markdown_toolbar_button({ icon: "table", data: { "md-tag" => "| header | header |\n| ------ | ------ |\n| cell | cell |\n| cell | cell |", "md-prepend" => true }, title: _("Add a table") })
   - if show_fullscreen_button
-    %button.toolbar-btn.toolbar-fullscreen-btn.js-zen-enter.has-tooltip{ type: "button", "aria-label": "Go full screen", title: _("Go full screen"), data: { container: "body" } }
+    %button.toolbar-btn.toolbar-fullscreen-btn.js-zen-enter.has-tooltip{ type: "button", tabindex: -1, "aria-label": "Go full screen", title: _("Go full screen"), data: { container: "body" } }
       = sprite_icon("screen-full")