Remove plugin-manager from the list of core plugins

Plugin-manager wasn't a core plugin in Gerrit v2.16,
therefore it should not be expected to be included in the
release.war and returned in the list of core plugins to install.

Change-Id: I33d05851c7e4b454b132fd117017c43fcef3be01

src/test/java/com/googlesource/gerrit/plugins/manager/repository/PluginsRepositoryTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/manager/repository/PluginsRepositoryTest.java b/src/test/java/com/googlesource/gerrit/plugins/manager/repository/PluginsRepositoryTest.java
index f24b154..7d43f0a 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/manager/repository/PluginsRepositoryTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/manager/repository/PluginsRepositoryTest.java
@@ -36,7 +36,6 @@
           "commit-message-length-validator",
           "download-commands",
           "hooks",
-          "plugin-manager",
           "replication",
           "reviewnotes",
           "singleusergroup");