Remove plugins we don't use on Admin.

These are a few plugins that I don't think we use on admin anymore and should be removed from this list.

playbooks/roles/jenkins_admin/defaults/main.yml

@@ -94,9 +94,7 @@ jenkins_admin_plugins:
   - { name: "matrix-auth", version: "1.2" }
   - { name: "matrix-project", version: "1.3" }
   - { name: "mailer", version: "1.9" }
-  - { name: "ldap", version: "1.10.2" }
   - { name: "pam-auth", version: "1.1" }
-  - { name: "ant", version: "1.2" }
   - { name: "build-user-vars-plugin", version: "1.3" }
   - { name: "credentials", version: "1.15" }
   - { name: "ssh-credentials", version: "1.7.1" }
@@ -105,7 +103,6 @@ jenkins_admin_plugins:
   - { name: "parameterized-trigger", version: "2.25" }
   - { name: "multiple-scms", version: "0.3" }
   - { name: "git", version: "2.2.2" }
-  - { name: "thinBackup", version: "1.7.4" }
   - { name: "maven-plugin", version: "2.5" }
   - { name: "copy-project-link", version: "1.2" }
   - { name: "scriptler", version: "2.6.1" }