Remove plugins/plugin-manager job as it defines its own job

In Change I71bfad7a the plugin-manager plugin defines a job for itself
since it requires the release.war built for its tests.

Change-Id: Ic8b32bb63795f8d0567d578e56b105df54ab05d3

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index b35aa05..7205947 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -57,12 +57,6 @@
         - gerrit-plugin-build
 
 - project:
-    name: plugins/plugin-manager
-    check:
-      jobs:
-        - gerrit-plugin-build
-
-- project:
     name: plugins/replication
     check:
       jobs: