Prune dead links

source/_presentations/automated-testing-test-driven-development-drupal-8.md

@@ -7,7 +7,7 @@ events:
     date: 2018-06-27
   - name: DrupalCamp London 2020
     location: London, UK
-    url: https://drupalcamp.london/training/Automated-Testing-and-Test-Driven-Development-in-Drupal-8
+    url: ~
     date: 2020-03-13
   - name: DrupalCamp NYC
     location: New York, USA

source/_presentations/drupal-8-php-libraries-drupalorg-api.md

@@ -21,7 +21,7 @@ events:
     joindin: https://joind.in/talk/14851
   - name: DrupalCamp London 2019
     location: London, UK
-    url: http://drupalcamp.london
+    url: ~
     date: 2019-03-03
     time: "12:05 - 12:50"
 ---

source/_presentations/tdd-test-driven-drupal.md

@@ -19,7 +19,7 @@ events:
     location: London, UK
     date: 2017-03-04
     time: "16:15 - 17:00"
-    url: http://drupalcamp.london
+    url: ~
   - name: DrupalCamp Dublin 2017
     location: Dublin, Ireland
     date: 2017-10-21
@@ -41,7 +41,7 @@ events:
     date: 2019-03-02
     time: "14:00 - 14:45"
     location: London, UK
-    url: http://drupalcamp.london
+    url: ~
   - name: NWDUG
     date: 2020-05-12
     location: Manchester, UK