🔥(backend) remove code related to export pdf docx

The export is managed by the frontend, so we
don't need the code related to the export
in the backend side anymore.

src/backend/demo/data/template/code.txt

@@ -1,10 +1,2 @@
-<page size="A4">
-  <div class="header">
-    <img width="200"
-      src="https://impress-staging.beta.numerique.gouv.fr/assets/logo-gouv.png"
-    />
-  </div>
-  <div class="content">
-    <div class="body">{{ body }}</div>
-  </div>
-</page>
+<img width="200" src="https://impress-staging.beta.numerique.gouv.fr/assets/logo-gouv.png" />
+<br/>

src/backend/demo/data/template/css.txt

@@ -1,20 +0,0 @@
-body {
-  background: white; 
-  font-family: arial;
-}
-.header img {
-  width: 5cm;
-  margin-left: -0.4cm;
-}
-.body{
-  margin-top: 1.5rem;
-}
-img {
-  max-width: 100%;
-}
-[custom-style="center"] {
-  text-align: center;
-}
-[custom-style="right"] {
-  text-align: right;
-}