Remove handling of excerpt for projects (#591)

Hugo displayed a warning that the way that we used this
in our templates did not actually work:
```
WARN  calling IsSet with unsupported type "invalid" (<nil>) will always return false.
```

We only used this in one project though so I think it should
be fine to remove it alltogether.

Author: k-nut

content/projekte/2011-12-31-ernteteilen.md

@@ -4,7 +4,6 @@ lab: [berlin] #needed for Aggregation on Lab-Page
 imgname: berlin/ernteteilen.png
 title: Ernte Teilen
 showcase: true
-excerpt:
 
 links:
 - url: https://ernte-teilen.org

content/projekte/2014-05-01-portable-linked-profiles.md

@@ -4,9 +4,6 @@ lab: [berlin] #needed for Aggregation on Lab-Page
 imgname: berlin/portablelinkedprofiles.png
 title: Portable Linked Profiles
 
-excerpt:
-  PLP proposes a pattern for the creation, storage, listing and representation of public profile data.
-
 links:
 - url: https://github.com/hackers4peace/plp-docs
   name: Dokumentation

themes/codefor-theme/layouts/partials/project-preview.html

@@ -24,7 +24,7 @@
       {{ end }}
     </ul>
     </div>
-  
+
     <div class="card-body p-4">
         {{- if (and (not (eq .Params.imgname nil)) (fileExists (printf "static/projects/%s" .Params.imgname ))) -}}
         <img class="project-image" src="/projects/{{ .Params.imgname }}"  alt="Screenshot {{ .Title }}">
@@ -33,16 +33,7 @@
         {{- end -}}
       <a class="text-dark no-underline project-link" href="{{.Permalink}}"><h3 class="title">{{.Title}}</h3></a>
       <div class="text-dark description" >
-        {{ if (isset .Params "Excerpt") }}
-         {{ if (gt (len .Params.Excerpt) 0)}}
-            {{ .Params.Excerpt | truncate 200 }}
-          {{ else }}
-            {{ .Summary | truncate 200 }}
-          {{ end }}
-        {{ else }}
           {{ .Summary | truncate 200 }}
-        {{ end }}
-
       </div>
     </div>
     <div class="card-footer border-top-0 bg-white d-flex justify-content-between">