[REM] component: remove t-keepalive directive

We don't see any usecase for it, it makes the code more complex,
and there were still potential unresolved concurrency issues with
it.

Part of task #295

tools/playground/app.js

@@ -443,7 +443,6 @@ async function start() {
     owl.utils.whenReady()
   ]);
   const qweb = new owl.QWeb({ templates });
-  debugger;
   owl.config.env = { qweb };
   const app = new App();
   app.mount(document.body);

tools/playground/templates.xml

@@ -31,7 +31,7 @@
             t-att-style="topEditorStyle"/>
         <t t-if="state.splitLayout">
           <div class="separator horizontal"/>
-          <TabbedEditor t-keepalive="1"
+          <TabbedEditor
              js="false"
              css="state.css"
              xml="state.xml"